I tended to keep the normal A times- especially after 6 months.. although you'll probably find they're not as sensitive to A time changes, they get less affected by OT. UT continues to be a pain though!
Try not to stress about the A times- i know it's hard, but you know now what you need to do. You might look at every 6 weeks or so adding 15 mins or so- but they vary.. I found up to about 7/8 months they really built up the morning A time till it was quite big, then once they start crawling they get more tired so need to nap earlier. I would suggest to just keep his A times as they are, then as soon as you get a nap refusal one day, stretch the next day's A times by 15 mins. This is your cue that it's not long enough. I also found that by 6 months or so you know your LO pretty well and if they're going to go down for a nap or fuss because they are UT. If that happens, don't fuss about trying to get them to settle, just get them up again, play some more- say 20 mins or something.. then go again. At least if they are over tired you can then have a chance of getting them to sleep. You will NEVER get an UT baby to settle! Not in my experience anyway!