Top Grafting Apples

Oaknut

5 year old buck +
I have a number of vigorous volunteer apple trees that drop early that I want to top graft to late hanging varieties like Kerr. What is the best height to graft them at? I was thinking about just a bit higher than deer can reach, so I don't have to worry about fencing them. But maybe low enough so the deer can help me keep growth from the original tree pruned off. Any suggestions? Thanks.
 
I have done a few chest high. I have left them unprotected and the deer have not found them.
 
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