generally a good start is with how tall the person is, however with a backed bow and even self bows you can go shorter depending on your skill tillering.also a longer bow will be more forgiving,which could help out on snap shots wingshooting. i would probably over build it, and get the 55#@25 like you want, but make sure itll pull to say 28" safely. that way if he decides to show the great bow his brother made to his buddies you got no worries about someone over drawing it. you could also do a search on bamboo backed ipe on here and check out some other ppls builds to get some ideas