You want to ensure you can see the compass from standing at the helm so adjust your height accordingly.
NOTE: Installing ANY plotter at the helm like this WILL cause interference with your compass. Any time you do, you should have your compass professionally swung and a deviation card made. I have yet to find a plotter that could not be swung, even magnet door plotters like those from Raymarine, Garmin and Simrad, Lowrance HDS units etc..
RANT:I think magnetic doors on plotters are simply STUPID and highly ARROGANT and IGNORANT move by the designers. However, the reality is that I have yet to see a compass like this that could not be swung, and a deviation card made, even when mounted in close proximity.
TIP: For those located here in Maine Dave Witherill of Pathfinder Compass does exceptional compass work. You can contact him via his cruising blog.
Pathfinder's Blog (LINK)