I know most of you don't have a spare pedestal kicking around, to do mock ups with, but with careful on-boat measurements you can easily figure and calculate your needed height. Here you can see why this factory OEM Edson guard was simply too short. The mounting plate is where I needed it to be in order to have good visibility for the compass and this left too little room for a hand hold..
You will need to measure to the top of the compass dome and then add about 4-5" clearance between compass top and the bottom of the mounting plate. This height will vary slightly depending upon your physical height.