First and I have repeated this ad nauseum, YOU DO NOT TENSION THE SHROUDS TO SOME % OF THEIR BREAKING STRENGTH. It may be true that most shrouds are tensioned to between 10-20% of their MBL, but this is not how you set tension.
There are two goals with rig tension, the first is to induce proper prebend. Basically how much the face of the mast bends backwards over the length of the mast, from the hounds to the top. Prebend really changes based on the cut of the sail and the conditions, but most cruisers set it for 15kn of breeze. Ideally you reduce prebend for light air, and increase it for heavy air, but many cruisers just don't bother, or have a hydrolic backstay required to change it on the fly.
Anyway, after the amount of prebend is set, then the only tension to apply is just enough to keep the mast in column while sailing upwind. That's it. How much tension it takes just depends on the boat. It doesn't take a professional, and it isn't that complicated.
0) set prebend based on the cut of the sail
1) at the dock get the top of the mast centered by snugging up the top shrouds and working your way down the rig.
2) once the mast is centered make sure all the shrouds are snug, you can put a gague on them if you want, I don't bother. If you do just set them to the same tension.
3) go sailing and beat upwind in 5-10kn of breeze. Ideally in flat water
4) while beating look up the rig, is it still in column? If so you are done, but it won't be. So take in two full turns on the leeward shroud.
5) tack the boat
6) take up two full turns on the leeward shroud (So the shrouds on each side match)
7) look up the mast, is it in column? If not then take in two more turns on the leeward shroud
8) go back to 5
Once the mast remains in column on each side, take a tension reading on the shrouds. Then equalize them. Take a 1/4 turn off of the higher and add 1/4 turn to the lower. Then remeasure and confirm the mast stays in column. If it does then you are done, if not then you may have a slight imperfection either in the mast but placement, the mast hounds, or the chainplates. Track those down if you want, or just retension until the mast stays in column on either tack.
Congratulations your boat is now tuned for those conditions! We have a base number from which to work.
Next time you go sailing in say 15-20kn of breeze start at #5 and retension the boat for the higher wind speeds. It will take more tension, but how much again depends on the boat. Probably in the range of 1-2 turns.
Personally I don't use a loose gague, because I don't care what the tension is, I use a set on machinist calipers to measure the distance between the rigging screws. This means I can always get back to my neutral numbers even if I forget how many turns I have added or taken off.
The average shroud tension for moderate breeze is going to be somewhere around 10-15% MBL is mentioned above, but I know boats that sail with tension as high as 35% and as low as 3% of the MBL of their shrouds.
For a more detailed look speak with a sail maker, North has a good write up, and PS
Boat Clinic: Tuning the Masthead Rig - Web Only Article has a good one. Ideally of course your boat has been raced a lot and the class has a tuning guide for the boat. This is where tension gauges are great, because rig tension is the only way to exchange tuning numbers. 150lbs of tension on my J-22 is exactly the same as on your j-22, but 3.275" (stud to stud measurement) on my J-22 port cap shroud has no relation to yours.