I suppose this would all make more sense if we can visualize what I think is going on. In the table below, I took the temp and resistance factor values for 480 SS off steam engine. I then used my coil Ohm as a starting point to calculate the actual Ohms that the mod would see at the given temps. I then calculated the Ohm difference from step to step and the % change from base (same as the resistance factor). I then took the calculated coil ohm readings and added in the constant resistance from the deck to get the "system" values.
The graph on the left is what, I think, everyone is telling me is going on. The mod sees a higher resistance because of the deck, but at the end of the day it doesn't matter because the resistance sill rises the same... just starting from a different point.
The graph on the right is what, I believe, really matters and why the graph on the left is irrelevant. The mod uses TCR to calculate the % change of resistance needed to hit a target temperature. And when you add a static, non-changing value to your reading, it throws off the percent change.
The mod uses TCR to figure out what % change is needed, starting from the base resistance at 68F, to hit the target temp. So, if I set my mod (as it is now) to 392F, it multiples the starting resistance of the system .245 by 1.2484 and ends up with 0.305858. Using the table above, that puts the actual temperature of the system at somewhere around 490F because the mod will not read 0.305858 Ohms until the coil is contributing 0.240858 of that value.