The tuning guides Vic mentioned are the way to go. But I will add a few points. The tension reccomendations are just starting points you can be very happy and safe with looser or tighter settings. Reason is the boats are not all built the same, some masts are much stiffer than others, the chain plate locations are different from boat to boat, some are 6 inches more forward than other boats. A few dont even have the chain plates symetrically located port and starbpoard. So dont worry if you end up with a reading of 35 or 45 they can all work.
best to to get the top of mast centered first. Tuning guides may cover this , what I do is put a steel tape measure on the main halyard and hoist it to 6 inches from the top, For reference points measure from the center of the bow fitting to a point just aft of the shrouds and put an equal mark on the top of the toe rail port and starboard. then with with shrouds just barely firm measure to the mark on the toe rail using equal pull with your hands on each side. I shoot to get within an 1/8 inch. From hear follow the tuning guides and when adjusting the upper shroud always do the same changes to both sides. so your mast stays centered up top. count your turns. When all done you should be able to pull on any one stay and see the top of mast deflect, then pull the opposite stay and the top of the mast should deflect exactly the same amount but in the opposite direction. Just using tension numbers will not keep the rig centered.
In my mind symetry is just as important as tension, and without symetry you get very confused doing the visual fine tuning under sail, which is what you are after. If happy with your settings, buy a cheap set of calipers and measure the turnbuckles, right it down and then you have a base setting to start with every year. You can also tape the threads of the turn buckles to remember how much to tighten them in the spring