For once shorter is better.

More survey results from Fly Carpin.

This truly is a huge piece of the carp puzzle. It's not always possible to get within 20 feet of a feeding carp, but I try hard to get inside of that range before I make a cast.  Dapping is ideal if I can get away with it.

Actually I believe this principle holds true in most types of fly fishing. I know those long casts in A River Runs Through It look awesome, but if you really want to catch fish - wade closer, cast shorter.