I usally tip a guide around 20 percent (usually more). As others have said the productivity of the day isn't something I tip on (well beyond his/her control). I do however tip on how well they did in putting me on what fish I did catch and just generally how well they did in ensuring I enjoyed the day. While guides do receive a percentage of the guide fee, if they are working through a shop, they do not recieve the full fee. So if a full day trip costs $250 the guide probably sees less than half of that amount. In addition he/she likely provided flies, water, lunch, etc. out of that fee. As many people tip on the quality of the food/drink the guide may have spent as much as $20 of the guide fee just on "extras". That really isn't much for a days work. As a result I do feel that a generous tip is warranted if the guide did a good job with those elements of the trip that are within his/her control. I personally use the same guide on my yearly trip to Colorado and as a result feel that tipping him a bit more is in line as every trip I've ever had with him has exceeded my expectations and he is always willing to fish as long as I'm willing to regardless of how long the trip was "scheduled" for.
Just my .02
Brian
"My father was very sure about certain matters pertaining to the universe. To him, all good things - trout as well as eternal salvation - come by grace and grace comes by art and art does not come easy." Norman Maclean, A River Runs Through It