Hmm.
Should not one be (if we are talking about an ideal trim) aiming for a particular STABTO value rather than %MACTO?
For a given Laden Index TOW the %MACTO will vary according to the aircraft weight and therefore would be at different positions under differing weights and therefore affect economic flight.
Isn't the idea to ensure that the C of G is aft of the Centre of Pressure so that less STAB TRIM is used (thus decreasing drag) to stabilise (balance) the aircraft in flight.
The STAB value being determined through %MAC values at the particular weight of the aircraft.
So if a particular STABTO (a constant)induces less drag, then as the weight would also be a constant in one example, the variable is %MACTO.
Therefore as one cannot sensibly aim at a moving target (%MACTO), it would be wise to target a STABTO value instead and not worry about the corresponding %MACTO value.
In my experience most Boeings like a 4.5 STABTO value.