I know that the 1/4 per foot is standard but for the toilet I had read that 1/8 inch is good, especially for 4" pipe (this is 3"). Supposedly any solids in that are flushed might not flush as well as the liquid would drain faster, leaving the solids behind. But I have also seen that claim disputed. As I said, right now it is at 3/16 over 18 inches and I guess I should raise is just a hair. I haven't glued in the top sanitary tee or vent pipe yet. That's why it looks so clean! But thanks.