I agree that it is terrible. The most professional way that I've found to work around this (PC version) is to enter the item and quantity as I normally would. Then right below it, I enter the same item but in the QTY I enter a negative number. This doesn't help for percentages, but if you do a Buy 2 Get 1 Free type of situation, enter a Qty of 3 on the first line and enter a Qty of -1 on the next.