I'm not good with the math either. What I do is draw it all out on paper when changing the size of anything. I'll draw out a whole quilt, add all the changes in inches, add up the inches to come up with total fabric, make sure the length and width is calculated -- it all has to be right out in front of me before I'll even start. I started doing that after I started a new lap quilt with new sizes, left it for a bit, went back to it later, and went straight back to the original directions. It was a mess. And I didn't have enough fabric to fix it, so I had to "unseam" the blocks I did first, re-cut them to the smaller, original pattern size, re-sew them, and then add them to the rest of the top (yes, I finished the top with the original directions, never noticed I was doing it all wrong until I looked at the whole!) It took me forever to do what was supposed to be a quick top. After that fiasco, I draw out and create a new pattern when I want to change, and put away the old pattern, so I don't ever do this again!
I feel your pain!