I get What you mean.It is a function of how many beakers you have accumulated at that point and how many are required to get a breakthrough. Say you have to have 2000 beakers for a given tech and you have 1800. You need 200 more and make 50 per turn. That means 4 turns needed.
If you add 3 beakers via scientist, it will do nothing. 53bpt is not enough to alter the turns needed. It may change how much cash you get when you turn down the reseach on the last turn, if you can.
Now if you needed 180 and where making 56 and make two more scientist to get to 62bpt, you would shave off a turn. Of course only C3C get 3 bpt for a scientist.
So With Vanilla civ 3 the scientists arn't as effective. (Because I have Vanilla Civ 3).