Ahhh, the dark art of setting uppers.
My understanding is;
Pull them tight to depower by prebending. (some people also do this to flatten sails in a drifter).
Let them off to depower by giving less support and more gust response. Pulling the cunny very tight will flatten the sail adequately and will hacve a combined effect of serving to slacken (take load off) the uppers by bending the mast.
Ideally you will let the cunny off downwind to allow the uppers load up again and support the spinaker.
Side effect of slacker uppers is you lose the ability to point, not necessarily a detriment to VMG but you just loose that high mode.
[I once forgot to pull them on at a gusty RS800 open. We couldnt point high upwind and got lee-bowed a couple of times but managed to win the race by still being able to sail a fast low mode with good gust response. Fortunately the mast did stay up downwind

]
Somewhere in the middle were you are providing sideways support but without to much aft bend is max power.
12L on the latest type spring loos gauge is my lucky number generally. (Need to measure without main on - any cunny tension makes them slacker.)