Melissa - I'd say go early. I've found that 1h 40 naps are OT for us. She's already done 5.5hrs before her nap right? So I'd probably aim for no more than 5hrs after her nap so maybe 6.10pm latest. Its so tricky b/c that's not going to give you a much later WU meaning a long time til nap tomorrow, BUT she will likely be better rested with a 6pm BT than if you push her to 7pm.
We've just put DS to bed (7.30pm) & I am rather scared as although he has huge bags he didn't really seem all that tired. Eek!!!
I am hoping this might give us a slightly later start tomorrow & I might be able to use the opportunity to push his nap a bit later. If WU is as usual, then nap will be 11.10am at the absolute earliest. I am being brutal about this, but I really think we HAVE to get his nap shifted to after lunch if I want to stand a chance of a) him making it to a normal BT without OT and b) him waking up later in the morning.
ETA: Melissa - was also going to suggest what Sara just said. As I just wrote above, maybe the answer is the same for you too. Pushing the nap later will help shorten the A time to BT reducing OT, OR mean you can achieve a later BT, both of which should help you get a later WU.xx