Start with 3.15, 3.30 would be a big increase. I would let your naps tell you when you've got the right A times -- they should be more than an hour, most likely 1.5 hours or more in order to be restorative naps. I'd say that you need some longer A time in order to get those longer naps. Those nap lengths make me think he needs more time -- if they get dramatically shorter, like 30 minutes and he's waking crying then you've probably gone too long. There's some fine tuning that happens at this point to get it right. I think that if you get some more restorative naps, he won't be quite so OT at BT and then he'll sleep a bit better.