"mono sect" wrote
My boat pulls to the right with a clockwise
spinning prop, which direction should the fin
thing be turned to compensate

Assuming you're talking about an outboard or IO, the fin thing, which some
people call a trim tab, is like a little rudder on the back of the drive
mechanism. It will turn the drive by exerting some force on the back of it
just like a rudder would turn a boat. The confusing part is that you're
turning your drive, not your boat, so you want to adjust the "fin thing" to
turn your drive in the appropriate direction to push your boat the way you
want it to go.

and how much?

Just enough.