Any level spot at least 2 feet from electric motors, large hunks of ferrous metals (iron, steel, etc), large current-carrying wires, or other sources of interfering magnetic fields. If you have a small hand-held compass you can use it to check there is no deflection in the desired spot. After that it is just preference. I put mine at the rear corner of the boat, and it paired with the TM and calibrated fine. It's not likely to get stepped on there, and is out of the way.