Member GerhardRP did quite a bit of work on modeling. Maybe that would help you:

http://www.diyelectriccar.com/forums/showthread.php?t=39931

As you try to understand my modelling, start with this formula:

field(I)=(V-IR-Vb)/Speed.

If you make a bunch of measurements with various currents obtained by varying voltage and load, measuring motor voltage, current and speed using SI units, you will get the motor "constant" which is a function of I. Notice that you don't need to know the torque for this procedure.

Measuring the resistance is a little tricky, because it is so small, and the brush voltage confuses things. Accurately measuring the starting surge current and voltage or using a briefly stalled motor can do the trick.

For starters, use maybe 2 volts for the brush drop.

After you have the field as a function of current, you can predict torque (ignoring magnetic losses such a eddy currents) using

torque(I)=Field(I)* I, again in SI units.

Good luck,

Gerhard