total timing reading accurate with no load?

The reason for advancing timing with rpm is there is less time in miliseconds for the burn to build pressure to get the most leverage.

But with increasing rpms, combustion conditions often improve, so advance can be slowed down as burn speeds up.
see The New Distributor (Session 136) from the Master Technician's Service Conference

The purpose of vacuum advance is provide more lead time for lower density mixtures.