The tension of the diamonds all depends on the cut of your mainsail.
Also, the amount of rake on your spreaders are important. But if you have raked the spreaders back 2-3 cm, you should have 5-7 cm of prebend in the mast.
Stretch a string from the mast top and down to the rotator arm, measure the distance between the spreaders.

This gets you going, but you must study your main while sailing and adjust the prebend/spreader-rake accordingly. If the main becomes to deep, you need more rake. You should be able to flatten it completely with the downhaul and main. If you can't, you probably need more prebend.

What type of mainsail is it (sailmaker and style) and what battens are you using ?

Good luck.