Frank,
I think the usual advice for fractional rig is that the rig should be tensioned enough to give a few inches (half mast thickbess for & aft perhaps) pre-bend. This is then held in check by the lowers, whilst also ensuring a straight mast (athwartships) when sighted up the mast groove. This probably sounds drastic, but this bending of the mast is important to prevent the mast bowing (panting) forwards in a chop when mainsail is not set - potentially resulting in mast collapse!
I know that adequate tension was crucial to windward performance with a Seal 22.
I have been advised that door to heads on Super seals will jam when rig is too slack or too tight - could be a useful indicator (so long as you're outside)!!!
I think Kemp sails have a leaflet on this - possibly info on their website. I think there may also be basic info in S26 handbook (mine's on board at moment).
Phil.
Phil Longhurst