I've never used a guide for hunting, but I do scuba dive and it is pretty similar. I generally give a 10% to 20% tip depending on the service provided. If they are friendly and enjoyable to be around,they get toward the higher end of that. If they are grumpy and not so fun they get toward the lower end of that. I've dived with an outfit in Fiji where the owner was our dive guide and that was really enjoyable and he got a tip just like any dive guide would have.