The old theory was to put the tang about 30" above the hounds, and then folks started going nearly masthead -- and we witnessed a lot of mast breakage. The Robert's idea of 1/3 makes a lot of sense, depending on the mast.
For example, on my Taipan 4.9 the tang is only 30" above (flexible mast), while on the Hobie Wave it is nearly masthead (stiff, non-bending mast. )

The reply above this one says the loads are higher than spinnakers, but that is mainly going upwind or on close reaches. Off the wind, it is about the same. And when you are going upwind or close reaching, the main is always sheeted in.., or should be.

It would seem to me there are two ways to go about the project: get the pole lower or have the sail recut, but I would not go really high up the mast.
Good luck,
Rick