I recently picked up a new (to me) Colnago with a triple ring crankset and 8 speed cassette. I find it most comfortable (in flat Florida) to keep the front in the middle chain ring and that puts me in a comfortable riding speed (18-20mph) at about the middle cogs in the rear (so the chain is basically straight). If I move to the larger ring I start leaning towards the bottom half of the cassette (cross chain) and if I use the smallest chain ring I lean towards the top end of the cassette (again, cross chain) unless I am going down or up a significant grade. So for flat ground I try to just keep the chain as straight as possible and maintain an average speed as close to 20mph as I can (I am the limiting factor on speed, not the bike or gears). I do this mostly because it keeps the chain straight, which I've been told is ideal, gives me a comfortable cadence, and good speed with lots of room to move up/down the cassette before I'm crossing or having to switch chain rings.