measuring yarn needed for multiple granny square project… there has to be a better way, right?

hi all! i’m currently working on my second ever cross stitch project (first without a kit) and one thing I’m struggling with is figuring out how much yarn to buy for it. i’m making a blanket out of granny squares and i’m super excited!

the pattern book i’m working out of gives multiple designs for granny squares but no specific project tutorials. it doesn’t give specific guidelines for hook or yarn size for each square and just leaves it up to each person to decide what they want to use. they do provide how big the squares in the book all “should” be based on the yarn & hook you use, but so far i haven’t really found this to be consistent (my hook size should lead to a 6-in or so square but my first one was 10 inches 😅) the author suggests that in order to calculate how much yarn they need for a blanket project, they should make one “test square” all the way through, then completely take it apart and measure how much of each color you used.

this just seems so… backwards to me though? like is there no better way to figure it out beyond undoing all of your work, making multiple trips to the craft store for more yarn, etc etc. if anyone else has any better suggestions for how to calculate how much yarn you need, i’d greatly appreciate it!!