'77 Johnson 115hp.

How do I set the trim tab vane on the lower unit?
It's easy to physically loosen, set, and tighten. However, what I'm asking
is where to set it, how to know if it's the right setting, etc. I seem to
remember it's some kind of trial and error thing, but I don't know what to
look for. Please give me a quick crash course in the care and feeding of
that thingee.