Can you fly to a one foot accuracy no matter which "mark" you are looking for?
The key is for everyone to be using the same Altimeter setting whichever is chosen as the Reference and stick to the correct Height or Altitude.
Years ago we had three BarAlts on our helicopters and no RadAlt.....and in Norwegian waters the Field SOP in the Ekofisk was based upon RadAlt Height which the Helicopter Service aircraft had in addition to their BarAlts.
They did not provide QFE settings as they used QNH for their BarAlts.
Thus a small problem in traffic separation standards.