OK...so lets say you need 10 blocks. In each of these 10 blocks there will be 2 sets of strips.
I don't know about you, but I cut my strips WOF, and generally, you have 40" to play with. You'd need 20 sets of strips to have two in each block...you follow?
Now, how long are the strips sets in an individual block? Let's say they're 6", you can get approx 6 set of 6" strips from the 40" WOF (6x6=36). You need 20, so you'd need four sets of long strips to get that - 6x4=24 (which will actually give you 24 small strip sets).
Sooo, let me try to generalize...
strips in ONE block x number of total blocks = how many strips you need
WOF / length of small strip = # of small strips from wof
total # of small strips / # of small strips from wof = # of wof strips you need
width of strip x # of wof strips = yardage required
clear as mud?