Here's I do the math:
for a 8 x 10 layout with sashing going around the outside
9 vertical sashing strips per row X 10 rows = 90 vertical sashing strips
11 horizontal rows of sashing (includes the top/bottom outside edges) x 8 strips / row = 88 horizontal sashing strips
90 vertical + 88 horizontal = 178 sashing strips needed.
Is the 4" square finished (cut at 4 1/2") or unfinished (finishes at 3 1/2")?
Assuming that the 4" square is the finished size (cut at 4 1/2"):
Each sashing strip needs to be cut at 4 1/2" x 2 1/2". You can typically get nine 4 1/2" strips per width of fabric (assuming usable width of fabric is at least 40.5").
178 strips / 9 strips / wof = 19.77777 --> so 20 wof are needed to cut the 178 sashing strips.
Each sashing strip needs to 2 1/2" wide (assuming 2 1/2" is the unfinished measurement and they are finished at 2")
2 1/2" / wof * 20 wof = 50" of sashing fabric needed to cut 20 strips at 2 1/2", with each strip making nine 4 1/2" x 2 1/2" sashing pieces.
50" / 36" / yd = just a tiny smidge over 1 3/8 yard, so get a minimum of 1 1/2 yards of sashing fabric
As for the cornerstones, you will need 99, not 90. (11 rows of horizontal sashing with 9 cornerstones per row).
At 2 1/2" cut size, you can typically get 16 squares per WOF (assuming at least 40"). You need 7 rows of fabric to cut the 99 squares. At 7 rows of fabric by 2 1/2" in width, you need just a smidge under 1/2 yard, which I would round to 5/8 yard cornerstone fabric.
Good luck!