Mechanical timing is only going to get you so close... and as things wear the timing will shift yet again so I would really aim for the best average of all speeds.
The advance setting that works on flat ground may ping the motor up a hill... that's why the vacuum advance helps so much... on flat ground, high vacuum, lots of advance. Up a hill low vacuum, a little less advance.
You can get a 'fuel economy' vacuum gauge and drive that.. its pretty much what you want to do but in a nice color coded gauge.
You *could* look at the old Model T' and A's... they had manual spark advance so you could dial in the motor.... but with vacuum advance and mechanical/spring loaded dist weights much of the 'easy stuff' is already taken care of.
