I think it depends on the time of the nap.
If it's a bit earlier than usual time for second nap, then maybe let her sleep longer (eg to her usual WU time from nap).
If it's later though, then maybe just let her have her usual length and then she'd still probably be able to last the shorter A time til BT.
How was BT in the end? What time did you go/are you going for?