Excluding charter trips in the salt, I've only hired a guide twice, and neither in this country. So take this for what it's worth. But, my $.02.
I'd go in figuring 10% min, 20% max, and considering 15% the middle ground. If the level of service was below my expectations, 10%, if it met, 15%, and if it exceeded, 20%.
Note that level of service is not the same things as success. I can be happy with service even if we had a crappy day, or not happy despite a great day. If he worked his butt off trying his best to show you a good day, and seems like he knows his stuff, then I'm happy. It's fishing. Bad days happen.
But if you get the impression he doesn't know what he's doing, or is mailing it in, or worse, doesn't listen to you regarding the experience you want, then I'm dissatisfied.