I owned and raced a C25 w/ backstay adjuster for many years. The purpose of the backstay adjuster on a C25 is to enable you to instantly change the tuning of the rig from very loose, for light air and sailing off the wind, to taut, for sailing to windward and in stronger winds. You could accomplish much the same purpose by easing or tensioning the main and jib halliards, or you could readjust the tuning of the rig with wrenches, but the backstay adjuster does it better and much more quickly.
Never over-tension a backstay adjuster, because it can do structural damage to the boat, and it isn't necessary for it to serve it's purpose. It's purpose isn't to enable you to put an extraordinary amount of tension on the backstay and forestay. It is to enable you to quickly change
the amount of tension from that which is normal
for sailing downwind or in light air, to the amount of tension which is normal
for sailing to windward or in stronger winds. It is the ability to change
the rig tension quickly
that makes the backstay adjuster so useful on a C25.
The only tuning guide I have found for a C25 with adjustable backstay can be found near the end of the article at: Catalina - Capri - 25s International Association
It worked great for me.
When you are racing and adjusting the backstay tension on the fly, you can't, as a practical matter, get out your Loos gauge and measure the amount of tension. You have to learn, through experience, to judge the amount of headstay sag visually, and by the amount of force that you exert on the backstay tensioner. It might sound like guesswork, but you'll find that it really isn't difficult with a little practice. Think about it this way: firm up the headstay when sailing to windward and in stronger winds, and relax the headstay when sailing off the wind and in light air.