this is what i've always used & it comes out perfect.
http://www.homemakingcottage.com/sew/cribsheet.htm

also, she has a equation for other sizes -
If you are sewing a sheet for a mattress of a different size, determine your yardage as follows:

length = mattress length + twice mattress depth + 8" width = mattress width + twice mattress depth + 8"
i used that to make a fitted sheet for DH's rack on the ship (navy).

here's one of the crib sheets i made out of sandi henderson's fabric from that -