davy

Like Frank mentions, there is the "magical factor" 2.5 which is valid for a 4% surface O2 consumption and 1:10 bellows which matches my situation very well.
This results in a quite simple formula that can even be used on-the-fly.
On verification during actual dives it was spot on at predicting pO2.

The pO2 drop is actually the partial pressure of the inert gas / 2.5
to know the pO2 during the dive, just subtract the pO2 drop from the pO2 you would have on OC.

I created a little excel sheet to easily play with it and learn.
the user interface is not that great, but if you want I can send it...

Also there is the V-planner which incorporates a full blown decompression planning for pSCR which works well.
Very cool to compare "actual" profiles Vs. OC reference plans.