I just keep a couple numbers in my head. A charm pack is 3/4yd (8 per row WOF, 5 rows =25"), a layer cake is 3yds (4 per row, 10 rows = 100") and a jelly roll is 3 yds (40 strips at 2.5" = 100"). Then I can think about how many different fabrics I want to use, so how much of each. It is also a quick way to see if a 'sale' is actually a good deal.