If you spread hps over body mass, it surely isn't more than 1%, more like 0.1% or even less. And then there is not even anything vital inside, which would probably lower this even more!
I probably wouldn't assign damage (considering how abstracted the DnD damage system is). Rather I would probably give a -1 or -2 penalty to actions involvingthat hand until it had healed up.
-1 or -2 is a bit much for just loosing one finger, to me... bleeding aside I'd ignore something like that from a mechanichal standpoint... mabye a -1 for a while until they got usted to using their hand without it... won't hurt using a longbow, for instance, if you're drawing the arrow back with it.
No permenate HP or DEX lose. It is only a pinky. There would probably be DEX damage for more important fingers and perhaps permanent DEX loss. Then again, I have pretty useless pinkies that don't get much exercise outside of typing. I imagine that there are some cases where you might assess a circumstance penalty to certain skill checks involving manual dexterity for a few weeks. Typing for instance.
One point of damage or maybe 3% of the total h.p. which ever is higher (there is a certain ammount of shock and pain involved). Also, it IS a bleeding wound, and he takes 1 damage per round until he stablizes (10% chance by the straight rules) or recieves first aid.