I think you could get way more than 3 blocks out of a FQ - FQ = 18x22 - blocks finish 4.5" square [with outside seams] - which if all you did was cut squares, a FQ would give you 16 squares that size plus and extra 18x4" strip. Now for a two fabric block you will have many many 1/4" seams [1/2" per piece in both directions-N-S,E-W] - say because of that you have to allow an extra square of fabric per block - this would still give you 8 blocks. There are 225 sections to the DJ divide by 8 - you get 29FQ. I don't know if it is written anywhere, but I would guess that is pretty close to what you'd need, translated into yardage would be 7yd of the same background fabric - I bet that would still give you tons of leftover scraps and possibly some larger sections though. I actually used a charm square with some off white fabric to make a block last year and had scraps left over!