Using plain old air, I would have (depending on the track, the day, the corner of the car) an 8-10 psi increase in the rears from cold to hot and a 6-8 psi increase in the fronts. With pure nitrogen, how much of a pressure increase should I expect from cold to hot? Note that a practitioner's answer is fine. No need for a review of Charles' Law or the relative density of water and steam. I'm just looking for a reasonable starting tire pressure for my first session with the nitrogen. Many thanks.

My experience with nitrogen shows I can start ~ 2 lbs higher than with air. YMMV...

Depends on how much air you get out (and its moisture content) before you put in the nitrogen (hard to get it to 100% and dry), so 2-4 lbs is a reasonable range.