I don't put as much emphasis on results (fish caught, size of fish), but instead ask myself three other questions when reaching for the wallet at the end of the day:
1. Did the guide teach me a new fishing technique that I enjoyed (e.g. not pulling plugs for steelhead) and will use again? If not, was he at least willing to have me fish my preferred method (vs. the easiest way to put fish in the boat, side drifting with gobs of roe in the US in the old days)?
2. Did I learn new spots to fish? I will often book a multi-day trip with the intention to self-guide after the first day or two. Most of the time, I'm on foot. A guide that ditches the boat and takes me to hike-in spots I can revisit on my own has earned a substantial tip even if I'm skunked. (Note that this can backfire, as guides are a lot less likely to show you their best, foot-accessible spots if you're going to compete against them later in the week. That's fair enough. It also helps if you're not a local.)
3. How hard did the guide work? Did we start early and end late, and was he on time?
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If you're fishing with the same guide over a week, the fish catching will be a function of the quality of the fishery and the skill of the guide (plus maybe a little angler skill thrown in). If it's one day, it's a crap shoot how many fish you catch.