new quilt pattern

Having basically completed the QFM (quilt for me), I need a new project. I've started a quilt for my mother, which she knows about, but obviously I can't post pictures here of that project. I want it to be a surprise!
So now I'm doing math (which is not my strongest skill) to figure out how to make a stacked coins quilt. I am inspired by this quilt, but I've bought enough patterns lately and I want to figure out how to do it on my own. Wish me luck!