I've RTFM for the TLC model and it seems like the value of Vdd is only used to calculate the output MOSFET on resistance which varies with the supply voltage.
It looks I'm doing it the right way, run the simulation, find out the supply voltage, run it again, paste the calculated supply voltage into the model's properties box and run it again and paste the newly calculated supply voltage into the properties box. The second guess is always pretty accurate.
The on resistance only matters when R1 or R2 are set to low values, when they're both 50k, it doesn't make any difference.