Final A on one nap would likely be around 5hrs as well. Perhaps 4.5 or 5.5, but roundabout there
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You shouldn't need to cap the nap at this stage really.
How are the nights after 5 1 nap days in a row?? I must admit it does look as though he needs to give up the 2 nap days completely as you are right 2 teeny naps will make him more OT than one nice restorative one.