Also, why do you have to keep replacing the sender?
Good question. In my limited experience, a quarter of them are bad out of the box. They're grounding out and when you put an ohmeter to them they just read 0 at all levels. On top of that, I've had a float get stuck so that it was reading a constant F. The other two times I've just been replacing the sender with an in-tank fuel pump (for an EFI LS swap), and then going back in there again to get the gauge to read better.
When you put your order in, order the float that
@FLCJ8 listed - it should be worth the investment.
Full is maybe a little higher than the circular mounting face, at least it is on the 20g tank, but in any case the float doesn't go above that very easily and probably shouldn't.
What I would recommend is first getting a depth for your tank. I just put a measuring tape in there and measured from the bottom to the mounting surface for the sender. For the 20g tank that's about 11.25". For the 15g it's going to be more like 9"? Luckily, the 15g tank is more like a square box than the 20g, which has a slope at the bottom. Once you have a depth, you can divide it into quarters on a piece of posterboard or whatever. The sender has a couple of bendable tabs that you can use to change the range of the float swing arm. Start by getting F and E ohm readings, and adjusting the F (full) tab so that your ohm reading is about 12 when the float is all the way up. If your float is below the level of the mounting face, you can bend the arm so that it hits a line just at that level. Next, drop it down to E and bend your E tab so that you have a reading of about 72 ohms. Your float should be just above the bottom depth of the tank, too. Then go through 1/4 and 3/4 and see what they're reading. Most important I think is what it's reading at 3/4, because like I mentioned the most common problem seems to be a F reading until it's about half empty, then dropping very quickly to E.
I think that's a good start - if you have any questions don't be afraid to ask!