Even though it might say "pre- shrunk" on the end of the bolt - I would still cut off a pice about ten inches longer than you think you will need - measure it - and then wash the piece as you will when it is a sheet. Measure it again.
If it stayed the same, great! If it shrunk, then now you will be able to place your corners where they should be.
I do not like to wrestle with a too-small shhet.