Pretty standard tipping around the board, and basically just like you tip in a restraunt (15-20%). Always tip more if you really enjoyed the day with your guide. If he works hard but fails to produce a fair amount of fish...well that could of just been a poor fishing day and don't let his hard work to find the fish that you did find go unoticed.
I like to receive tips from the client themselves and not secondhand from someone at the lodge, there are interanl rats even at the very finest lodges out there, very rare but it does happen, and it always a nice gesture.
for the two of you $50 a day is a very fair and gracious tip...$100 would not be that uncommon for a great day with a great guide.