I tend to judge how much to eat on the bike based upon what I expect to do in the next 20-30 minutes. If I am just spinning along at a moderate pace I would sip on an energy drink mix. If I am headed for a long climb I will eat a gel 5 or 10 minutes before I get there. Once over the top I will have 1/2 of an energy bar that has some protein. For me I always do better if I am ahead of the curve for food. As has been said in the past, drink before you are thirsty and eat before you feel hungry or down on energy.