My way might be painful but it should work for you. I figure everything based on square inches. 10 x 10 block is (roughly) 100 square inches (but you need a tad more to make it 10.5 in. for an unfinished block. Figure out how many blocks total you need to make the size quilt you want. Then figure out how many of which colors of blocks you'll need. Multiply number of each color block by 100. For example - you have 5 red blocks and 5 blue blocks you will need 500 square inches of each color. I use a calculation of 1440 sq. in./yard (36 x 40), you would need a bit less than 1/2 yard each of red and blue. I'd probably by 3/4 or 1 yard just for shrinkage and errors. It's tedious but it works. Then I add a bit for kicks and giggles. Sounds like your Mom's stash might the be place to go shopping. Save $$ and it's a gift from her to the senior center. Good luck with your project.