It's not so much a case of how long the last A time 'should' be, because all LOs are different. Some won't go to bed unless they have a long last A, mine (and others) like a super short last A time. It's more a case of finding something that works for your LO. As it stands you have a very short first A compared to the very long last A - and that would be fine if the naps were great and you weren't getting the earlier morning WU - but taking earlier WU (shorter night) into account plus the second nap being a bit hit or miss I would just even things up a little bit, they still don't all need to be equal. So, I suggest just leaving BT where it is (assuming this has been his BT for a while, even before the DF drop), shift the rest of the day on and see what happens.
I know you said he started waking earlier when you dropped the DF, at this age I would expect him to be able to still do a full night without waking from hunger so my guess is the earlier WU isn't necessarily the DF drop but more likely the first A being too short, the last A a bit too long and possibly a bit of OT by BT.
It's a bit of guess work, but that's my guess.