It is a bit of a balancing act, judging the wind down and settling time
I think Tracy says to allow 15-20 mins, but tbh if it took my LO that long to settle I knew I had got the timing wrong! I would pick a time, say 1hr10 and put down for the first nap at that time consistently for several days and see how long it takes him to settle, and length of nap. If you keep getting 40 min naps, try increasing by 5 mins for several days. If you start getting OT naps, you'll know you've pushed him too far.
if he was UT would he be able to finish the nap on me at all? Or is he just not transitioning?
It could be that he's not quiiite tired enough to transition, but still tired enough to sleep longer. Or it could just be a transitioning problem, very common for the next couple of months. If the A time seems right and naps are still 40/45 mins, it's probably developmental - so you could try wake to sleep.
The catnap is often the trickiest - can you AP it somehow?