It is true that the nida unclean period of time refers to sex.

It is true that the nida unclean period of time refers to sex.
[That is for the menstrual period a woman can not have sex.]

We love Avraham

But also it refers to the two major things that all uncleanliness refers to; that is- entering the holy Temple in Jerusalem and eating the meat of sacrifices. Also the sacrifice that a woman has to bring after the nida period is not for a normal nida. It is for a woman who has seen blood not during the regular nida period. There is a argument of how to compute this between the Rambam [Maimonides] and the Ramban (Moshe ben Nachman).[All rishonim go like Nachmanides including the Rif!]
This note is getting too long so let me just make this as fast as I can. Step 1: a woman sees one spot for one day or for a whole seven days. [The number of seven days is explicit in the Torah] She is a nida. She goes to a natural body of water on the night after day seven and she is clean. {A swimming pool can't work because nowadays it is constructed of hard cement and can be lifted whole out of the ground, and that makes it a vessel. It is not a kosher mikveh from the Torah itself.]

Step Two: If she has not stopped after day 7, but continues into any three consecutive days from day 8 until day 18, then she is a “zava”. And then she needs to count seven clean days. After those seven clean days she goes to the mikva or any natural body of water, and she is clean. She needs to bring a sacrifice; however nowadays, she [a zava] can't bring a sacrifice to Jerusalem because of two basic problems: (1) No Temple. (2) No red heifer.

  And she can't bring a sacrifice anywhere else because after God set his name on Jerusalem, Jewish people can't bring sacrifices elsewhere. This is all from straightforward verses from the Torah.

If she has seen only from day 8 to day 18 for one or two days (not three consecutive days) then she is a small zava. Then she goes to the mikvah that night and is clean.

I am writing here the opinion of the Ramban, not the Rambam.

There is a good reason I am not writing down here the opinion of the Rambam (Maimonides).The reason is a basic contradiction that so far I have not heard resolved. With the Rambam we all know that after day 17 you are supposed to go back to counting days of Nida right? Well there is one place where it is clear in the Rambam that you don't do that. [The place is where he brings the laws if one mixes up the bird sacrifices. So far I have not heard anyone mention this contradiction and I am too busy with other stuff to spend time on it right now.

So for all the women that have waited and been patient for me to get to the basic point here it comes: You need to wait seven days and go to a natural body of water. That is all there is to it. Women nowadays don't see for more than seven days so the whole issue of Ziva is simply not relevant.

[Just for the record I should mention the Rambam : a woman  has a regular time of thirty days. From the first time she has established this time she counts 7 nida and 11 ziva. And she just keeps on going like that. 7, 11, 7, 11, ... The meaning of this is that a woman could see during any one 11 day period and then be a Zava if she see three days straight!--even though then was no nida period that preceded this! . Besides that in the chapter about mixing up the bird sacrifices [based on the last tractate in the Talmud] there is a Rambam that seems to contradict this.]

There are people that count seven clean days in any case and they don't do it because of the argument between the Ramban and Ramban. They do it because of a statement in the Talmud which might mean that. But to me it makes sense anyway just because of this argument between these two rishonim. I mean if we go by the Rambam, then almost any time a woman sees blood could be ziva unless she has been keeping track according to the order of the Rambam. And if that is the case then she needs seven clean days.
Seven clean days means to check before the night of day one. Then to check on the first day  and then again on the seventh, and then to go to some natural body of water on the night after the seventh day.
[I heard one person suggest one should check on day 4 also which makes sense-- because if one misses day 7 and gets mixed up and it turns out to be day 8, then the whole seven days starts again if one did not check on some middle day.] [I think if I recall that that was at Torah VeDaath in Brooklyn that I heard this.]

In any case, you can see that there might be some reason to count seven clean days. Not because of that Gemara about counting seven days, for a drop which could be anything. The Bach says it refers to the color of the drop. But the basic opinion of Maimonides gives a decent reason to wait a full seven days since no one counts 7-11-7-11--... from the first time a girl sees. So that leaves us with the fact that any blood might be during the 11 day period.
In any case, you don't need to be strict like the Rambam. Going like the Ramban is also OK as far as I can see.

There is the question of יושבות על כל דם שבעה נקיים. But like I said the Bach says that means on all types of blood. It does not mean to add to the law of the Torah that all one needs is seven days.  And in particular, I should mention we are not talking about a rabbinical decree but rather a custom. If on rabbinical laws where there is doubt we go by the lenient opinion all the more so with a custom.

My basic approach to sex is that it must be on Friday night after midnight. [But this is only when it is possible for her conception to result.] This idea I got from the prayer book of Rav Yaakov Emden and I learned that even in my first yeshiva years before I got married. Reb Friefeld gave me that sidur.

For the sake of completeness: If let's say, we start with the woman and she sees blood. Then  she goes to the mikveh on the night after day seven,- even if she saw all seven days. She does a הפסק טהרה i.e. she needs to check. But then let's say she sees again on day 8. Then she is a small ''zava'';- she needs to wait the whole of day nine and then goes to the mikveh on the night after day nine. Let's say she sees day 8 and 9. Then she waits the whole of day 10, and goes to the mikveh on the night after day ten. If she sees day 8, 9, and 10 in a row, she is a "zava gedolah" and this is the case the Torah requires seven clean days.

What is a mikveh? A natural body of water like a river. In the Ukraine and Russia, these tend to be cold in the winter. You could take a ax and cut through the ice, but doing this at night is harder than doing it during the day when at least you can see what you are doing.  The trouble with mikvahs is the problem that they are often either made of cement that is hard enough that if you picked up the whole thing it would be picked up in one piece. That makes it a vessel. [That makes it not kosher דאורייתא] The other trouble is the plastic that they put under the cement as they do for swimming pools. Even if you don't have the first problem then there is always the second.  I am not trying to be strict. These are just two simple problems that often people are unaware of.  The way to go into a cold river is this: You leave on all your clothes except the shoes. Then dip your feet in the water. Then go out. The take the pants up to the knees and dip in again. etc.You don't take off the shirt or blouse until you are up to the arms.  Another way is to go in with all your clothes on and change them after you have gone out. Having them on helps a lot to not feel the cold. You in any case need to get home immediately and take off all the wet clothes as water has a high specific heat constant. It absorbs a lot of heat.