Matching forestay sag to luff hollow .....
There is one and only one tension in a forestay (headstay on a cutter, etc.) that will CORRESPOND to the 'luff hollow' - the long smooth curve that the sailmaker cut into the luff. The luff hollow is cut from the front panels to compensate for the EXPECTED sag in the forestay.
If the forestay is not precisely tensioned, then the angle of attack attainable from the sail's geometry is lessened. Same applies equally to: too-tight and too-loose.
Too loose a forestay and the angle of attack (ability) lessens, draft goes aft, and the CE .... moves to leeward !!!!!!! Result: aggressive heeling, keel begins to SKID off to leeward or minimal LIFT development, s-l-o-w-s.
For PRECISION matching the luff hollow TO the correct forestay tension:
(If you dont have the sailmakers original values of luff hollow). Take the jib and lay is on clean & FLAT ground, make a 'bifold' along the luff so that the luff section is FLAT on the ground with ALL wrinkles out of the luff section... the 'bifold' will allow the partially spherical sail to lay flat along the luff .... the mid & leech sections will have some wrinkles.
Then pull a string taught between the head and tack connection, then from the taught string put a few (perpendicular) marks all the same distance from the string well 'into' the luff. I sew down a long 1/4" wide (grey) tape along these marks .... all the way down and a few inches back from the leading edge of the luff. You can use a few 'dots' of 'permanent marker'. If you have ANY QUESTION during racing, etc. of correct backstay (forestay) tension, simply walk forward and put your eyeball near the tack of the sail a "look up" - if the added 'luff hollow stripe' is STRAIGHT .... then the forestay tension is ABSOLUTELY TOTALLY PERFECT .... and you cant get it any better because the luff section of the sail is EXACTLY at the shape that the sail designer cut into the sail. When 'looking up', your eyeballl must be perpendicular to the 'side' of the luff ... not from behind like when sitting in the cockpit.
Set the forestay tension CORRECTLY (depends on conditions, sail loads, sea-state, etc.) so that the SAG in the forestay EXACTLY matches the luff hollow hat the sailmaker cut into the sail ..... and the sail's luff shape will be fully optimized. The boat will point like a banshee, the keel will LIFT, and the wake of the boat will be coming straight out the stern of the boat, etc.
Warning --- dont overcrank on the jib winches as this will also increase the forestay sag, etc. ... if so (as indicated by the luff hollow stripe) then you need to add more backstay.
This is a simple VISUAL way to get correct backstay/forestay tension each and every time out that does not involve 'guesswork'.
hope this helps.