I usually have mine somewhere in the vicinity of 20cm below the centre of the bridle but it really doesn't have to be that exact.

The end of my pole is set up with wire strops with some 2mm spectra on the end to attach to the bows. On the beach I attach the mid pole rope to the bridle then loosely tie on the pole ends. I hoist the kite and then adjust the 2mm spectra ends to the bows so that the kite luff is at the right looseness (now I'm being really technical ) for the prevailing conditions. To be perfectly honest its pretty much the same most of the time anyway.

The height of the end of the pole will depend on the luff length of your kite and the hoist height on the mast.

Then if there is too much bend in the pole (or not enough) I adjust the bridle strop to suit.

Hope this helps!