I like this book.
Amazon.com: Sail and Rig Tuning (9781898660675): Ivar Dedekam: Books
Just found something similar online
Both describe how to get tension from seeing how much the wire rope
stretches over a 2 meter length, see page 29. They also give max tensions you should ever use as a percent of breaking load.
While the pressure gages are supposed to be notoriously off, mine checked out in the ball park comparing to the rigging
stretch. Somewhere the specs for your backstay adjuster
will say how many pounds of force is generated for how much pressure. (You can also get this from the piston area. Force (lbs)= psi times piston area.) You can use this number in your max force as a percentage of breaking strength.
I think the first book also says, if you're adding backstay tension and you don't see the luff sag decrease, stop. You've reached the point where you're just bending the boat for no purpose. This is secondary, if you haven't yet reached the max settings that they give.