Well, if you look here:
Average A times- BOOKMARK ME! you'll see that average A times for his age is somewhere around 1.20 -- but I'll also say that at his age, so much can change by the week, or even by the day as he gets older. And, of course, so much can mess with their sleep -- developmental changes, distraction, everything really!
How are you feeling about his tired cues right now? Do you find them to be fairly reliable? Do you get good naps off of them? I found at his age, that if my babies looked super sleepy, they would generally sleep for a good nap. If you've found an A time that seems to work most of the time, I'd stick with that and adjust it if he seems super awake at that point (maybe off of a good long nap before that) or put him down earlier if he seems tired earlier.
I'm guessing it would be somewhat ideal to shorten your day to closer to 12 hours -- but I also get that you have an older LO and I'm sure lots of other things going on, so if that's the best that works, that's what works.
Let me ask -- when you were getting 30 minute naps, did you find that he was waking up fussy from them or happy? Or was he just super alert and then fussy? Could be some overstimulation going on too (I found with older LOs, there was always a little overstimulation going!) causing your short naps. You could also try stretching earlier A times in the day, but not the later ones.
In your EASY above, I'm guessing that after the 40 minute nap, you actually may have been a little late on the nap and that's why he didn't settle right away in the sling.
If I were you, I'd probably stick with 1.30 A time and see where it gets you. Do you get the sense he needs a longer A time first thing in the morning or later in the day or he likes them all consistent?