Only a partial answer; my Perdix only displays CNS percentage, not minutes. From the manual:
The CNS percentage is calculated continuously, even when on the surface and turned off. When deco tissues are reset, the CNS will also be reset. The CNS value (short for Central Nervous System Oxygen Toxicity) is a measure of how long you have been exposed to elevated partial pressures of oxygen (PPO2) as a percentage of a maximum allowable exposure. As PPO2 goes up, the maximum allowable exposure time goes down. The table we use is from the NOAA Diving Manual (Fourth Edition). The computer linearly interpolates between these points and extrapolates beyond them when necessary. Above a PPO2 of 1.65 ata, the CNS rate increases at a fixed rate of 1% every 4 seconds. During a dive the CNS never decreases. When back at the surface, a half-life of elimination of 90 minutes is used. So for example, if at the end of the dive the CNS was 80%, then 90 minutes later it will be 40%. In 90 more minutes it will be 20%, etc. Typically after about 6 half-life times (9 hours), everything is back close to equilibrium (0%).
This mentions the same NOAA tables as in the blogpost you posted, so my guess is it will be factored in somewhere internally, just not visualized.
So basically it works similar to residual gas tracking; if you already have a residual (CNS %)value after the first dive, it will be deducted from your remaining percentage for the next dive.
probably not exactly the answer you were hoping for but hope it helps.
