If you do buy a loos gauge - a sound purchase - it includes (or did) a handy little tool for measuring wire size (in case you do not know for your boat), and a table that includes typical tension guidelines. That should provide you with a good starting point
Next time I am on the boat (it is raining now) I'll take a note of the tension on Selene shrouds and backstay. Can't do the forestay as it has the furler on it. Your mileage may vary, but it'll be a datapoint.
You probably know this already, but remember as you tension the shrouds and stays a key function of this rigging is to keep the mast straight and aligned correctly. We don't have much flex in our mast, so a bend is unlikely, but you could have all the tension right and have the mast leaning to one side! So go slow, tighten each turnbuckle in turn (i.e. don't fully tension one side, before you move to the other side!), and keep eyeballing things....
(BTW, I am assuming you have wire rigging; the tension is different for synthetic)