[Bott]

Well, from my experience and understanding, the gaff rig is much less a creature of tension as a Bermudian rig. Because of the lower aspect ratio, the gaff sail is much less dependent on the scrupulously maintained shape of the mast for a good airfoil shape. Also, the lower aspect ratio makes the force magnification of staying much less, so you get it from both sides...

That being said, there is nothing to be gained from slack rigging and a sagging mast.

Without getting into the raw numbers or sailing your boat for a season to learn what it likes, I would advise making the rigging as tight as it needs to be to make sure that the mast stays in column through regular sailing stresses. I doubt you'll need a tension gauge, just lots of sailing with a analytical eye. Ultimately you want your boat to sail efficiently, and efficiency generally means speed under sail.

Hope that's at least something to go on before someone who knows better than I can hop in and give better information.
-eric