Capt Joe,
Help me understand why you are having to subtract the 15'.
Since each unit has it's own antenna and works as an independent GPS, it should take you to the lon/lat cordinates that are marked; regardless of the where the unit is that marked the cordinate. So you should not have to subtract the 15'? In other words, if you give me the cordinates off of your unit I could take my GPS and go right to the that point even though my GPS could be miles from your unit. Since both of your units have their own antennas shouldn't they do the same?
If 2 units are sharing one antenna then that is a horse of different color.