Ted -
There is no 'short answer' yet only ONE correct set up tension for a forestay ... depending on the wind strength, depending on the accuracy of the jib winch handle, etc. etc. etc. .... It all has to do with how the head sail is designed and what 'cut' there is at the luff !!!!!
Allow me to bore you with the details. Setting proper headstay sag is probably THE most important adjustment when racing or precision sailing. Without such adjustment you can trim and shape until the cows come home but you'll only be guessing about the optimization of the sail shape and its trim if the headstay isnt properly sagged (or not). This is a loooooong explanation, so please be patient:
When a sailmaker designs a headsail thats either attached to a foil or is attached by hanks, the sailmaker ‘assumes’ that the rigging is at normal tension (usually about 12-15% of the breaking strength of the wire). Unless specified otherwise a ‘cruising’ sail will be designed for ~15kts. of wind and the sailmaker will expect that headstay to ‘stretch’ a wee little bit when the sail is wind-loaded at the design windstrength. When beating, the stay will also begin to ‘sag’ off to the lee-side of the boat ... usually quite predictably. This expected sag (aft and to the lee side) is ‘compensated for’ by the sailmaker cutting a smooth curved amount of sail material from the luff section of the sail - called “luff hollow”. If the hollow isn’t cut to this shape and when the headstay does sag all that material at the midsection of the luff would be ‘pushed’ towards the center of the sail and the designed point of maximum draft would move aft (increasing weather helm) and the amount of draft would get much deeper causing the boat to heel more ... sometimes causing even more apparent weather helm. Also, since the center of effort of the sail is now moved/sagged off to leeward ... its like someone moved that headstay several feet to leeward or radically somehow changed the sail’s angle of attack: the boat ... heels over, slows down, can’t point, and starts to ‘skid’ off to leeward ..... and all because the headstay sag no longer *matches* the luff hollow that the sailmaker designed into the sail !!!! Its all action and reaction. If the forestay sag exactly matches the 'luff hollow' thats cut into the front section of the sail ... thats PERFECT.
How to induce headstay sag (and make pointing ‘worse’): loosen the backstay; apply too much winch tension to a sheet (all that applied load to the jibsheet eventually ‘distributes’ to the *headstay* which increases the sag); sail in wind well beyond what was the target design wind-loading for the sail.
How to know when the luff hollow shape is matching the normal sag in the headstay:
1. Ask the sailmaker how much luff hollow was cut into the sail, and where on the luff up from the tack is that maximum amount. When on a hard beat and with a Mark 1 eyeball near the tack, look up along the luff and simply estimate that the ‘sag’ you see is approximating that value that the sailmaker gave. If too much sag, tighten the backstay or release some sheet tension; if too little sag, loosen the backstay or increase sheet tension.
If this is too approximate, there is a more precise way ... .
2. Take the jib/genoa and lay it FLAT on FLAT **clean** ground or floor and make an ‘accordian fold’ about 2 ft. back from the luff - the accordian fold looks like a flattened Letter "S" with the leech trailing out from the 'fold'. The accordian fold will allow the ‘curved’ 3D shape of the luff of the sail to lay FLAT on the ground. Work out ALL wrinkles from the luff section - you want the luff to be absolutely FLAT. Then take a string and pull tight along the curved luff shape .... what is ‘missing’ between that tight string and the FLAT sail luff is the ‘luff hollow’ that the sailmaker cut from the luff edge of the sail. Measure and record or REMEMBER that ‘hollow’ shape if you want that sail to take the ‘shape’ as was designed.
For easy bombproof ‘precision’, take that taught string and move it a few inches across the luff and so that its parallel to the ‘first’ string position but now laying over sail material. Then get some 3/8" or 1/4" wide adhesive backed ‘draft stripe’ material and apply ’straight as an arrow’ just behind the leading edge of the FLAT sail luff on the ground. When sailing and having ‘pointing problems with increased weather helm’ ... just take a walk forward and ‘see’ (with your mark I eyeball looking perpendicularly from the boats centerline). if that vertical stripe along the luff is dead-straight (headstay sag now *exactly* matching the ‘luff hollow in the sail’) the sail will be exactly at the shape that the sailmaker cut, the position of max. draft will be correct, etc. If that 'line' or hollow is not exact, then, do any adjustment necessary (backstay tension, jibsheet tension, running backstay, etc.) to keep that stripe ‘straight’ .... and your boat will now ‘point like a banshee’, with less heel, and wont be skidding off to leeward (and with the helmsman erroneously blaming ‘weather-helm’, etc.). Its very easy to overload a jibsheet on a large genoa ... and totally destroy the critical forward leading edge shape of the luff.
Want to race, quickly move to the side of a squall, point higher, or get to your far destination faster? ... **match** the headstay sag to the “luff hollow”.
So the only answer to the question of how much forestay sag? ... just enough backstay (or running back stay) tension so that the sag of the forestay ***exactly*** MATCHES the shape of the luff hollow that was cut into the sail. Anything less or more will not result in optimum shape of the headsail. Once the sag is matching the 'luff hollow' exactly ... THEN you can shape and trim the rig to perfection .... If the sag doesnt match the hollow ... then you will NEVER reach optimum trim.
hope this helps.
(If someone can advise how to upload pics *directly*, I can post such graphics that will more easily explain)