The thing to remember at this point is that you are making some big changes and things will not be smooth sailing for a few days. He will very likely have a shorter nap or a shorter night or longer NW. Don't worry as long as you are there with him and you stay calm, that is your key thing - do not join him in the frustration, you don't need to. use all your energy to be supportive for him.
With PUPD at this age it is just put down. Now, my DS could never settle back to sleep if he woke unless i picked him up so no matter how old he got he had a quick pick up cuddle and then down again, he settled very fast like that. Tracy's PUPD method I have never used properly myself (didn't need to) but I do trust that it is fine to just put down and stay. Hand on if needed, key phrase is very helpful too. I have repeated key phrases like a hypnotic mantra before and that seemed to be helpful.
You can pick up if you feel it is helpful or if he is very upset but you can also trust that being with him and using your hand and voice is supportive.
You know he is well fed so these WUs and not settling back are frustration and the change to how he is to fall asleep.
If
Is there any point when I must change tack? Previously I think 45min was a good time to stop
Change tack if you lose your patience. Nothing is worth you losing patience and getting frustrated or angry, if you are you need to either stop (take him out of cot if it's nap time and just start A time) or walk away, take a very deep breath and go back.
So long as you are calm you can continue for 45 min or up until the end of nap time.
The NWs are not going to stay this long - he may have been OT from the shorter nap and he will have been frustrated. it is going to settle down.
Do make sure he is getting his meals and milks and then trust he has no need for hunger in the night.
hugs too, you can do this x