Rig tension determines the SHAPE of the jib/genoa ... windloading on jibs/genoas is variable due to the changing and variable wind strengths.
purpose' of rig tension is not to keep the 'mast up' but rather to provide a structural 'platform' upon which to fly the sails ... and that requires quite precise tension - within limits.
There is not a SINGLE constant headstay/backstay
tension that is 'appropriate'; however, headsails are typically cut/designed
for the headstay/backstay to be @ ~15% wire tension when sailing in 12-15kts of windstrength
... and the 'sailmaker 'expects' the rig to be at 15% headstay/backstay tension when he/she designs 'stock' sails. .... and if you want 'performance' in other windstrengths then you should be adjusting the rig tension (on the fly via the backstay
) to match the SHAPE of headsail.
Here's how to set 'tension' to get the correct SHAPE of the headsail (should be quite close to 15% tension ... headstay/backstay/capshrouds for sailing in 15kts); If this is too complicated, then set the headstay/backstay/cap shrouds at 15% .... as thats probably what your (stock) sail were designed to be flown on: