Estimating mainsheet load?

[Rossynant]

I think this might be a gaff vs bermudan thing. On a gaff rig, yes you want plenty of downward tension, to help stop the gaff sagging off to leeward. So moving the traveller underneath the boom will let you do this, without oversheeting the main. In a bermudan boat, though, I am given to understand that in lighter winds you want to move the boom inboard *without* pulling down too hard on the end. I have a lot to learn about bermudan sail-shape [...]

Trouble is, the blocks I need to buy will be different for a single tackle vs an inverted W.

I think you will find principles similar and already familiar. Main reason for track was in fact to have sheet attachment moved to leeward, so to prevent twist - head sagging off. Naturally with the track there are many possible positions, so twist and tension can be controlled, as suits, indeed in light wind there is more twist to the wind itself and sail may be shaped to it.
As for blocks - I would buy a handful of simple, universal blocks anyway, also double. You never know, might come handy