I think we're possibly at a similar stage with this dreaded 1-0. We had a car trip to make at around 3.30pm on Saturday where DS fell asleep - totally the wrong time. We then had BT refusal, a late night, followed by OT NW's and an EW. I'm finding he needs a nap once, maybe twice, a week at the moment just to keep OT at bay, and even then I have to do a car trip if I'm going to have any luck in getting one!
The only way I know if DS needs a nap is if he's giving OT signs such as EWing, or if his behaviour is completely off. If he naps and a good night follows then I stick with no naps (and early BT's) until I see OT creeping back in.
It's very difficult. Can you keep her awake when you don't want her to sleep (in the car or buggy) with snacks or something?
I dont *think* that capping the nap any further will do you any favours tbh.