No, I don't think there is a perfect solution--one where all the outputs are equidistant. This would be a linear function, obviously, as all the switch setting values wold lie on the same line.

You can solve the linear system for individual resistors. For example, for a 4-position switch, using a single unit value for the resistor tied to Vcc, the resistor values (multiples of the unit value resistor) 2.86415, 6.69925, 14.28358 and 29.11765 all fall on the line described by the equation...

y = -x/33 + 31

where y is the switch settings (0 to 15) and x is the ADC value (>= 528.)

The problem is that the switch combinations (due to the parallel values of the resistors) don't give the appropriate value for y.

I think the lesson I have learned is that the circuit is good for only 4 or 5 switches when you have a 10-bit ADC. The R-2R circuit might give you 7 or 8 with a 10-bit ADC.