I suggest you either buy EQ or make a friend of someone who has it. I have done many resizings for friends. It doesn't take much time. On EQ you can decide what size blocks you want and see the over size of the quilt when you lay it out. You can change the quilt top or blocks whenever you want.
I just tried a pattern with 5", 6", and 7" blocks. Decided to go with the 6" blocks.
However, I used to be a math teacher years ago, so doing it the old fashioned way is OK too.