CofG,
That is correct but in modeling you also have to determine if the frame spacing of the drivers will accept the correct full driver diameter allowing for the flange diameter depth you decide to use. Many O scale locomotives have drivers that are a couple of scale inches smaller than the prototype to allow for flange clearance . For example, many PRR K-4s Pacific models have 78" drivers measured at the root of the flange to allow the drivers to be installed in a frame with correct driver spacing. The real K-4s locomotives have 80" drivers but are so closely spaced that they would interfere with each other on a model with typical O scale flanges that are 0.045" to 0.62" deep. All of this has to be considered when machining drivers for a model. Of course if you leave the flanges off of some drivers, then you can space the drivers correctly and also make the tread diameter exact.
The radius of curvature that the model will be expected to negotiate may also determine whether some flanges are eliminated on the model's tires. This was true on some real locomotives also.
Also lateral motion in the driver axles needs to be considered when fitting the drivers to the frame. I adjust lateral motion with custom machined spacer washers for each model that I rework.
These are decison you have to make before you start machining the drives for a model.
I just received a Lionel UP 4-12-2 to convert to 2 rail. This should be interesting.
Joe Foehrkolb