I'm getting ready to do something similar, here's my thoughts:
Don't boil the chocolate that long. It will end up kind of burnt/bitter from what I've read, add it late or in secondary.
Don't use sweetened chocolate, the oils can really make the beer screwy and the sugar will most likely ferment out. Use extract or cocoa powder.
Add what you think is a conservative amount of chocolate flavoring initially, and when it's time to bottle/keg if you need to add more you can. Not so easy to remove it if you go overboard.