I cut and run, then cut and run some more. LOL
Usually I cut one block, put it together and make sure it's right. If I've cut everything right and the directions are right then I cut all the pieces, label and put in baggies. That way I know if I need to go back for more fabric. Since I rarely make anything the way the actual pattern is I never know for sure just how much fabric I'm going to need. I suppose I could get one of those calculators but I think they're only as accurate as the operator so they probably aren't that precise. LOL