No Easy Answer
Rig tension depends on the usage. If racing there are ways to use rig tension to optimize sail shape and performance. On my Etchells we change rig tension based on the wind speed and seas and the sail makers provide those measurements. For cruising it does not matter too much as long as the mast is straight side-to-side and it does not move too much in sloppy seas. If you have too much weather helm, your mast may have too much rake so tighten the forstay and loosen the backstay. I keep my uppers at medium tension and my lowers just snug (i do not measure it in any way, just gut feel). It is good for the sails in most conditions that I see and keeps the rig in the boat.
