From memory the 8500c program for the UH-1H asks for an OGE hover height of 80 feet (could be wrong but I do remember doing them at that height).
I also remember the maint Test flight paperwork asking for the pilot to note any unusual vibes at an OGE hover height of 80 feet and an operating N2 of 6400 (normally 6600) I don't think I ever flew one in this config that didn't have a lot of unusual vibes!!