Sorry I missed this one earlier. "The first non-white strip" is difficult to define absolutely - what you're looking for is the equivalent of Zone VIII i.e. white with barely discernible detail, or 0.1 log D. This might be darker than you're currently aiming at.
If you find it easier to use a real negative to assess the highlights then that's fine - what you're aiming for is to set up the machine so that you get the results you want on a print. It's also important to be consistent across the grades so if you change grade, the resulting highlight density remains constant.
Hope that helps