Well, yeah, it's all relative to the size of your tank, of course. Just because my plant numbers are smaller, doesn't mean I'm not heavily planted.
50-70% sounds about right, for 'heavily planted'. If you're looking into the term 'heavily planted', for the benefit of your occupants -- such as Dwarf Puffers -- it's really all about breaking the line of sight. If you have lots of short plants and they can all see each other, being heavily planted won't help.