DHM...what a lay out! I've never seen DJ done that way. Here is a formula from Barbara Brackman for a sampler quilt that might give you some help. This is a 104x104 finished quilt. I'm not sure how to figure your background.
We'll start with the yardage for the 36 sampler blocks, each finishing to 12 inches. If you wanted to use just 5 fabrics I'd buy two yards of each. My theory is that it takes about 10 yards of fabric for a full-bed sized sampler so if you wanted to use 5 different fabrics:
10 divided by 5 = 2 yards each
3 fabrics?
10 divided by 3 = 3-1/3 yards each
10 fabrics for a scrappier look?
10 divided by 10 = 1 yard each
then there are setting blocks that take about 7 yards. Here is the link to her comments so you can see for yourself what I'm talking about.
http://austenfamilyalbumquilt.blogsp...t-yardage.html