It'd be helpful to know what kind of boat you have and what kind of
sailing you're doing... Ideally you want the pole to be square to the
mast for most extension and the clews of you symmetrical spinnaker to
be even and the curl of the luff to be about two-thirds of the way to
the top. However, you can't make that happen with a single attachment
point most of the time. In practice, if you have only one attachment
it will probably work best if it is about chest high when you are
standing next to the mast in the position you will take when gybing
(ie. on the foredeck of a keel boat or just aft of the mast in the
cockpit in a dingy). For smaller boats this will be good enough and
any deficiencies in pole height can be compensated by moving the
outboard end up or down. On larger boats you will need a track on the
mast and it must run high enough so that the pole can be swung through
the fore-triangle and, on a cruising boat, maybe high enough so the
pole can be stowed on the mast.

Good luck,

-- Tom.