there is already a thread that might cover your question
I’m not that knowledgeable yet I have read some, so take this with a Deer-lick of salt:
Height above the ground really shouldn’t matter overall. The higher your union I suppose the greater the chance for shoots to sprout off the rootstock, but not only is it relatively unlikely, you can simply maintain that with a clipper. Otherwise, it’s mostly a visual appeal thing. Some say the longer the rootstock the more of the properties that get passed to the scion but I don’t know if that is anything more than conjecture. I’d wager your bigger concern is making sure the roots are sufficiently buried that they form a strong base for your tree.
All of my grafts are are about 10-12" up the rootstock so that if they failed I could just nip it and try again.