In the absence of tension gauge, it's a fairly safe bet, once the mast is in column, to tighten the shrouds to "piano wire tight". There are plenty of guides online about the process for getting your mast in column, with the "right" rake or prebend (or non at all), and then checking and adjusting when underway to ensure that the mast is not sagging and remains straight.
"Piano wire tight" is good and tight but not overly tensioned. You want the shrouds to prevent mast movement and to keep it straight, but not overly stress the rig and hull
. Then when under sail check the leeward shrouds. They should not be under load but should not be slack to the point where they are floppy. If you have rod rigging you'll get differing opinions about how much wobble is too much...some riggers maintain that flexing of the rod will fatigue it over time, particularly at the heads. One of the best riggers here in Annapolis
scoffs at that notion, calling it nonsense.
Tuning can seem a bit like "whack a mole" since the tension on all the shrouds is related...tighten one, and others will likely need adjustment as a result. After a short time those relationships will become clear and you'll have a pretty good sense of what one change means in terms of other adjustments you need to make.
I would not get caught up in using a gauge unless you're racing
or want to extract every last degree of pointing when close hauled. Wire does stretch and the boat flexes, temperature goes up and down... I know racers who tune their rig before every race
, and it's interesting how much rig tension can change week to week even if they don't touch it. If the mast is straight when you're sailing and the rig is not so tight that it's stressing the boat, it's tuned well.