Neil,
Depends on a lot of things - length of headstay, size of the jib, strength and direction of the wind, and finally the headstay tension.
The sailmaker made your jib figuring on a certain amount of sag in the headstay. He put an arc into the luff, figuring that the sail would then set properly with that amount of sag.
On my Tartan 30, I tighten the headstay by tightening the backstay - the headstay doesn't even have a turnbuckle. Lacking a tension gauge, you can make a calculation of backstay tension by pulling on it with a known force and seeing how much it deflects. Or, hydraulics will give you backstay tension directly.
Sailing on the wind, increasing backstay tension will increase headstay tension, flattening the jib and allowing you to point higher. But eventually you'll get to a point where the leading edge of the jib is so flat that the helmsman no longer has a 'slot' to steer to. On my T30, this happens at about 1200# on the backstay - haven't done the geometry, but headstay tension will be somewhat higher - perhaps 1800#?
My actual headstay sag in inches?? No idea.
As for the roller furling, it will work better with tension on the headstay. You certainly don't want it floppy - makes the foil rub harder on the stay.