I usually figure on a usable selvage to selvage of 40 inches, so to figure out yardage you do this:
40 (usable inches from selvage to selvage) / A (block size) = B (# blocks you can cut selvage to selvage, always round down)
C (# of blocks needed) / B = D (Number of rows you need selvage to selvage to get your number of blocks needed, round up to nearest whole number)
D * A = E (number of inches needed cut edge to cut edge, round up to nearest whole number)
E / 36 = F (Yardage needed, round up to closest quarter)
Hehe looks complicated, but if you just follow the steps it's pretty easy to figure out yardage.
So for instance for your focus fabric you'd do:
40 / 6.5 (A) = 6 (B)
15 (C) / 6 (B) = 3 (D)
3 (D) * 6.5 (A) = 20 (E)
20 (E) / 36 = .75 (F)
So you would need 3/4 of a yard of the focus fabric.
Can you tell that math is my second language?? Of course the way I do it leaves some fudge factor, but I like fudge factor when buying fabric.
Cheers!
Rachel