I did the CN after her normal A time. At 7/8 months, she could only handle 1.5 hours after that or she would be OT at BT. Don't be worried about a late CN, you need to be quite flexible during this transition.
What I would advise is this -- Stick to a fixed WU time and BT. I don't know about you but I used EBT quite a lot after the transition to 2 naps, and I found that once the EWs started, EBT simply added to the issues. I decided to stick to a fixed BT and to keep a CN to get to it. So on days when the nap ended 2 or later, I would go for a one nap day. Otherwise CN (10 mins) and then bed at 7.