Originally Posted by dunster

With 10" finished blocks, the block's diagonal measurement will be about 14". Your 8" borders take up about 16" total i each direction, so you want your inside quilt measurement to be about 90-16 x 108-16, or 74"x92". To get the number of blocks needed tip-to-tip in the vertical direction, divide 92 by 14, which gives 6.5. You can either round up or down, so suppose you decide to round down to 6. In the horizontal direction you need 74/14 blocks tip-to-tip, or about 5 blocks. So far you need 6 rows of 5 blocks all tip-to-tip, but you also need to fill in the spaces around them. This will be an array of one fewer rows by one fewer blocks, or 5 rows of 4 blocks. So you need 6x5 + 5x4 = 50 blocks, plus setting triangles.

If you were to make the quilt with a straight setting you would need the same interior size, but the math is simpler. In the vertical direction to make 92" you can use 9 blocks (90") and in the horizontal direction to make 74" you can use 7 blocks (70"). This means 9x7=63 blocks, but you don't need any setting triangles.