I think this an area most tend to neglect when PT, most just check the gains and fail to notice the condition loss, check this and you can get a pretty good idea how many R/P you will need for the next session.
When i was doing PT on Kissi I applied them at circa 70% two at a time to maintain maximum gains to condition loss per pack, let them drop into the yellow, you either waste part of a R/P, if say the third takes him to over 99% or the second only tops him up to 85-90%.
So rather than just filling any vacant training mode I selected the best gain available while staying above the 70% bench mark for condition that I set.