The place where the roots begin should be just below the soil. I never go below maybe half an inch deep myself when planting Foxtails. I do not know if that depth that you stated will be to deep, but I would not keep it there. I plant some fan palm a little deeper, like Copernicia or Bismarkia because they can handle it.

I don't know that it would matter that much. As they get older the base of the trunk swells up and roots come out above soil level anyway.