if you use 2.5", you will have 640" minus seams
if you use 3", you will have 520" minus seams
if you use 3.5", you will have 457" minus seams
1. straighten cut ends before measuring, trim selvedges
2. formula to see how much you will get from any length of fabric:
fabric width _____ / strip width _____ = total number of strips _____
total number of strips _____ x fabric width _____ = total length of strips after sewing together _____
how long will the center strips be? (2, sometimes 3)
length of fabric (36" in this case) ____ x 1.414" = _____ (50.9 in this case)
the total length of strips you get is the same whether straight of grain or on bias, but you have more seams in bias binding as the strips get shorter because you move from the center toward the corners. but both center strips are more than 50" each so you'll have plenty with one yard of 40" fabric.