Okay, Geometry teacher here, let's see what I can do:
On a 45-45-90 triangle (where one side is a right angle, and the legs are equal), the sides are x, x, and x*sqrt(2). So, if I know that one leg is 5, the other leg is 5, meaning the hyp is 5*sqrt(2) or about 7.07.
So if you know the LEG and want to find the HYPOTENUSE, multiply the leg by sqrt (2).
If you know the HYPOTENUSE, then DIVIDE by sqrt (2) to get the legs. Ex: If my hyp is 7, each leg is 7 / sqrt(2) or about 4.95 (and if you think about this, this makes sense since it's close to what we figured out in the above one).
What I do, is I find out about how long the legs are, and then add 1/2" for seam allowance. It's not exact, but I'd rather have too much then too little. I have a bit of trouble working the seam allowance into my calculations because you lose part of the point, so I just add a little extra.
Hope this helps. Let me know if you need more