From a math burnout perspective, it's a right triangle, therefore a^2+b^2=c^2. So top and side lengths of your center square, squared, added together, will be squared length of your diagonal. take the square root of that and that's your length, add 7/8, or do what I (and most everyone else I know) do and round up to the next whole number, sew on, trim up. viola. Do this on a calculator. It's not worth it to go OMG MATH EWWW.. heheh. That would actually give you the proper TRIANGLE size but most people I know use a square then cut off the corners, which is a wasteful method, agreed, BUT.. you can make a presto chango viola HST from that corner you cut off if you just sew another seam while you're at it, 1/2" away from the first. My brain just flitted away but I think if you do the square method just find the midpoint length and add an inch then trim down.