I agree with Kate that the stimulation level has a big impact which is often not considered.
45 min naps are usually considered either UT or inability to transition from one cycle to the next (ie needs to learn to transition) but it can also be US (under stimulated) which can lead to LO not being tired enough even on a good length A time...just in the same way that OS (over stimulation) can lead to LO being either tired sooner and needing to go for a nap earlier than normal or if given the normal length A time resulting in what looks like OT behaviour (crying, nap refusal, short napping, in ability to settle).
My own DS liked a super short wind down and for his A time to be FULL of stimulation right up to the point of being tired, then a really quick nappy change,one song and down to sleep. If I attempted longer wind downs or low key activity he actually got pretty fed up and frustrated.