2" sashing (finished?) so you plan to cut your strips 2 1/2" wide?
if so---
you can cut 14 (2 1/2") strips from 1 yard of fabric- if your fabric is 40" wide that gives you 560" of sashing strips-
are you planning to put a 12" strip on each side of each block- then a long strip across the bottom of the row? or 12" strips all the way around with corner stones? are your blocks 12" or 12 1/2"?
if putting a strip on all 4 sides of each block (12") you can surround 11 blocks with some left over-
to figure out what you need decide on the cut width of your strips, divide that number into 36" (a yard of fabric) that lets you know how many strips you will get out of a yard-
then you need to figure out how many sashing strips you need & how long they need to be- divide the 40" width of fabric by the length of sashings you need to see how many you can cut out of the width-
( you can cut 3 - 12 1/2" sashings out of 40" of fabric {37.5"}...