I have a question about mounting height for BUG rating option (Trespass).

How can I determine “distance from grade” (per LEED definition of mounting height) if my project has varying surface level? Should I base my calculation on the actual height of installation from respective surfaces (road, hardscape, footpath etc.) or from a universal project reference point (eg. +0.00 external road level) or level of grading at lighting boundary)?

Please advise.