Make deriveDimension?

[bc0n]

That instead of redefining I really just want to write the equation and automatically define the 1(!) undefined Dimension.

 @makeDimension DiametralPitch PerInch # = Nteeth/PitchDiameter, the number of teeth per inch of pitch diameter, larger DP is smaller teeth; dp is not a pitch in the same sense as basePitch
  DiametralPitch(nTeeth::Int, pd::PitchDiameter) = DiametralPitch( nTeeth / convert(Inch, pd.measure)) # macro defines DiametralPitch{PerInch}(num), but this is the actual definition

[bc0n]

This is like the new @deriveMeasure and may be able to follow the same approach:
@deriveDimension DiametralPitch(1) = Number(1)/PitchDiameter(1), but does it need a conversion factor (1) or can those be omitted? @deriveDimension Number/PitchDiameter = DiametralPitch "dp" gives a better L-R read...

The other part is that many dimensions are compound, in that the constructor ideally takes arguments:
@makeDimension DiametralPitch PerInch # = Nteeth/PitchDiameter defines the type but the more correct & useful constructor is:
DiametralPitch(nTeeth::Int, pd::PitchDiameter) = DiametralPitch( round(nTeeth / convert(Inch, pd.measure)))

So the better approach would be @deriveDimension DiametralPitch(nTeeth::Int, pd::PitchDiameter) = DiametralPitch(round(nTeeth/convert(Inch, pd.measure))), where I just do the parsing and make all the things.
...let's punt on this for now until I'm using Dimensions more.