Most people use stainless pop rivets (monel if available) isolated from
the mast with a bit of durlac or similar to attach a ring. It's still
hard to come up with a specific height for your case because I don't
know how tall the kite is. If you have access to the boat you're
taking the sail from you could run a tape measure from the mast head
down to their attachment and make yours about the same distance.

When you say "relaxed" day sailing I'm guessing that you're talking
light winds so you may be able to get away with a slightly lower pole
position. Your boat is just on the upper edge of end-for-end gybing
size wise. Racing crews would certainly do end-for-end gybes, but a
short handed cruiser might opt for dip pole gybes and/or a sock. Since
you only want one attachment point you are stuck with end for end gybes
but be aware that as the wind comes up getting the pole made on the
mast will require a good deal of muscle. Having the pole attachment
mid to upper chest level for the person at the mast will make those
gybes as easy as they can be. For your service I think making things
easy is probably the most important factor. If you have a local sail
maker or sailing coach you might try and get them to drop by for a bit
of more specific advise.

-- Tom.