Hi there Jenn, I thought the same thing when they told me to cut the AM nap back. I thought no way, it's always been our best nap, it's also when I catch up on sleep from EW/NW. I was sure the Finn was going to reject that PM nap no matter what. But lo and behold he just can't get through the day without it and has done a PM nap every day since cutting that AM nap. I got one OT nap since starting a week ago, and that was because I got the middle A time wrong - and the OT nap was still over an hour so we could make it to bedtime, just. It's all up to the middle A time.
I would start at the 1 hour AM nap and then move that middle A time accordingly until he's doing a decent PM nap. But at the same time, I wouldn't be afraid to end that AM nap at whatever he gives you i.e. 35, 40 or 45 mins and just put him down sooner for the PM nap. I think you'll find that he will take a really good PM nap if you have the right A time.
Finn is a month older than Ethan but has always been on the low end of A times anyway.
Here is what I have found with A time:
Morning A before nap between 3 - 3.5 hours, no more than this and closer to 3 hours most days.
Middle A after 30 min nap = asleep by 2 hr 45 mins, after 40 min nap = asleep by 3 hrs, 45 mins = asleep by 3 hr 15 - 3 hr 30 (have to watch carefully here). It may not be the same for Ethan but it's what has worked for us so far.
Final A - can be up to 4 hours on a 2 hour nap but no more, closer to 3.5 hours. If his PM nap is only 1.5 hours I try to aim for 3 or 3 hr 15.
If he does 1 hour AM nap you will have to wing it and determine what A time will get you 1.5 hours PM nap.
I know it's hard but try not to go out while you are working this out. I mean of course you have to go out but just do your best to do it immediately after his AM nap (like be ready to go when he wakes and don't be out more than 1.5 hours) or at that period between second nap and bedtime. Once he gets used the routine I think you will be able to have a bit more flexibility with your middle A.
He will be grouchy but you just have to use your friend distraction :-)