I have no hard and fast rule either. I just buy what I like but I always buy at least 2 yards of everything. The one thing I do do is, when I find a fab that I love I pick a complementary fabrics to go with them. For instance, if the fabric is a medium, I will buy a dark and a light to go with it. That way I always have fabs that go together and I can fill in as needed form my stash. Hope this helps you. Enjoy your shopping!