Originally Posted by MadQuilter

It's geometry you need, not algebra, and graph paper will do most of the work for you. There are some great books out there that talk about starting with a center panel and adding borders until you reach the size you like. If you want to use patchwork in the border (for example flying geese) and the pattern doesn't quilt fit, then you can add a piece of coordinating fabric in the middle of the border panel and make it fit. Nobody is going to measure if your piece is 1/2" bigger on one side or the other (and if they do, then smack them). Yes, adding piano keys is a good way to create a border and those keys can be adjusted to make the border fit too.