I must have gotten lucky because I set up my FD-5801 with no real idea what I was doing and don't have any problems. Someone suggested chainstay length matters; my chainstays seem to be about 415mm. But I just looked at my bike and I'm wondering if seat tube angle matters too. The steeper the seat tube, the more horizontal the cage will sit, which means more of the chain is "covered" by the derailleur.