I think the "ideal" time is whatever works for your LO
Owen is 20m - we tired to switch from one to two SO many times and kept having to revert back to 10m and the AM and long nap in the PM. Finally I realized I need to do things slightly different. I realized he could be awake for approx 5h before he needed to have a nap - anymore tham 5h and he was overtired and the nap stunk. So regardless of what time he woke up, 5h from then on the dot - he was down for his nap. Also, I realized that some days he still needed a 10-15m cat nap in the AM - so I gave it to him on the days he seemed to not be able to make it til his PM nap. At first it was like every other day he needed the cat nap - soon though, it was every third day, then once a week and so forth til he didn't need it anymore.
Also, I didn't use early bedtimes all that much. He went to bed early if his nap was short and that's about it.
I still plop him in bed 5h from his wake time, regardless. Also I limit his nap to 2h15m- any more than this and he wakes early in the AM. So we have been on one nap smoothly for about 31 days now - it took a while to get here but being patient and flexible were how we got here.
We do:
645am wake
1145pm - 2pm nap
730pm bedtime
Anyhow that's my story! Hope that helps some, hang in there!
Zoey