trim tab vane; how do I set it?

'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.