There are 1584 sqin in a yard of fabric (36x44).
Layer cake with 40 pieces of fabric is 4000 sqin.
Layer cake with 42 pieces of fabric is 4200 sqin.
Charm pack (5") with 40 pieces of fabric is 1000sqin,
Charm pack with 42 pieces of fabric is 1050sqin.
Jelly roll (2.5"x44") with 40 pieces of fabric is 4400sqin.
If you divide these numbers with the sqin in a yard you will get fairly accurate yardage.
I hope this helps.