The first few weeks of a full term LO are quite sleepy, I wonder if your LO is still in that sleepy phase and so is ready for a sleep on the shorter A time? If he is sleeping well on the shorter A time then I'd go with that. If he is short napping on the shorter A time due to UT then I'd try to stretch it out another 10 mins.
Remember the guidance A times are only guidance, we all adapt the times to suit our LOs.
If you record your times each day for a while you might see a pattern emerge, for instance maybe a very short first A time with a slightly longer second A time and for the rest of the day, or the other way around with a longer first A time and a bit shorter for the rest of the day - not every A time has to be exactly the same throughout the day.
Although I am totally for starting a routine at this age (I started mine young) I would also say LO is still very very young so keep expectations low, don't panic if things don't appear 'perfect' and expect to take some time to get to know your LO and what is best for him, it does take time.
hope this helps