How do you know when to transition your kid to 1 nap?

My daughter transitioned to just a single nap soon after she turned 15 months. I found that she was resisting bedtime in the evenings, and was taking longer to fall asleep at night. This meant that she did not need the second nap. I dropped the mid morning nap. She would have her bath and play after breakfast and then would have her nap after lunch. (which I moved up by half an hour). This meant that she would still be fresh for outdoor play in the evenings and tired enough to sleep by 7pm.