Mounting height of the repeater is not consequential. The MXR10 should be located with easy access to the power and the coax feeding the antenna system. Ideally, the location should be clean, dry, and not exposed to direct sunlight.  The antenna mounting height is important as increased mounting height allows the signal to clear obstructions that will attenuate the signal. Antenna structures may have local association, zoning, and code restrictions or requirements. Antenna structures that exceed 20 feet near an airport or 200 ft in any location, need to be compliant with Title 47 CFR Part 17. This is regarding FCC and FAA Registration Marking and Lighting Requirements.