I had a queen size shrink enough that it did not reach to the bed skirt and it was 100 x 100. I think in washing that it shrank 4 inches in each direction. No way of knowing if it was the fabric or the batting that shrunk or dimpled up. So Now I make my queen size l04 by l04. But you need 60" for the top of the mattress and double the drop that you want on the sides so measure for sure.

Try a square 10x10". Measure after washing and this will give you an easy percentage to work with--if it's only 9", that 10% shrinkage.