I don't have the slightest idea of the relationship between the iron/constantine signal and its interpretation on the meter. I fully expect that it is nonlinear. The reason I think that is the fact that in the early seventies we had expensive, single purpose instruments that we used to get digital temperature readout. Had it been a simple, linear relationship, then the voltmeters of the time could have provided this with simple analog electronic conversion.
Now that a "computer" is available inside these meters, the sky is the limit as far as conversions and scaling. Since this thread is indicating that you can get a meter capable of thermal display for $19.95, it's pretty difficult to justify much messing around.
Best of luck,