I believe the highest home court I have ever seen in D3 (not that I have much experience) is C. There may have been a C+ somewhere, but I think it was C.
Greensboro in Tark is as successful a program as you will find, and it has only a C home-court advantage.
I also believe (and this has no backing other than my belief) that home court advantage isn't dictated by how well you do at home, but instead by your overall success.
I think HCA is designed to, well, provide some advantage for the home team (the spreads would seem to support this) but I'm not sure precisely how it impacts game play. It could be a flat amount based on letter grade, it could be a modifier of home team skills/talent, it could be something else.