I fall back on a method I learned long ago. Looks like fractions...... how big is the center piece? Let's say 6
10/12 x 6/? Cross multiply crossways, 12 x 6 = 72 divided by 10 + 7.2. The top two numbers of the fractions are the numbers you know, ie, starting with 10" pieces with 6" (?) in the middle. Then the bottom two numbers are where you want to get to. 12" pieces with ?? in the middle. Try that on a piece of paper where you can write the fractions out properly and cross multiply. It's confusing, I know. If you need more help let me know.