When dd1 was transitioning from 2 to 1, we'd do a really short nap in the AM (around 30-45 minutes) and then we'd get a good long afternoon one. So, you might try the short nap that you'll cap for the school run and then try for the afternoon nap somewhere around 3 hours A time -- of course that's just a guess from me, I'd try a few different A times and see what you get. We're actually in the reverse situation right now -- we have a school run in the afternoon so we're guaranteed to have a short PM nap.