Usually you figure how many your stitch number is divisible by - like 80 is divisible by both 8 and 10 and base your decs on that. So for 80 sts, you could k8, k2tog all the way around. Next round or row, no decs, then the one after that do k7, k2tog around. Then no decs and k6, k2tog, etc, using one less stitch between the decs until you're down to k2tog and then you just pull the tail through the remaining sts like a drawstring. For the last couple dec rows you might not to put a plain row between them, but just dec on 2 or 3 rows altogether.