|The last several times I've preshrunk cotton (preshrink it a minimum of twice, by the way), I've noticed the same two things:
Thing 1: The width shrinks significantly more than the length. Thing 2: (here's the fun bit) the width seems to shrink roughly 10% each time. I'm not claiming this is a hard and fast rule, but for me it works as a pretty good rule of thumb.
For example, a 60" cotton fabric would probably shrink roughly 6" in width -- so the final *real* width would be around 54". With this in mind, for cottons I usually get the amount of fabric the pattern specifies for the narrower width, or almost that much.