Here is the logic I would imagine I would use in "real life."
1. Find the range with regular AP.
2. If the first one or two regular rounds fail to inflict damage, load T.
3. If T fails to inflict damage, run like hell.
Of course other factors come in to play. For instance, the AI should never fire T at half-tracks or light tanks even if the first couple of rounds fail.
Also, I would model panicing crews. If my lonely AT gun is happily firing away at littl e old half-tracks and then a Tiger rolls up 200m away, immediately load up a T and fire. The crew would be panicing at this point.
Regards