if he falls asleep within the 15 min margin you can add that time to the nap. I.e. if he's put to bed at 8.30 and falls asleep 8.40 you wake him up at 9.25 (a 45 min nap). IF he's extremely cranky/tired you also have the option of putting him to bed as much as 15 min ahead of schedule. Got it? This can be applied to any nap!
Regarding the "winding-down time" this is absolutely normal. My son could lay in bed talking to himself for almost one hour when we put him to bed at night, quite happy
You don't alway fall asleep right away either when you go to bed, do you? As long as he's happy, let him be. The winding-down period is also a form of rest, but if you feel like it, apply the 15 min margin here as well.
What's important to remember is that you can't sleep for your child only serve him the basis for sleep
(the bed/a dark room/ the chant etc)! The rest is upp to him
Best of luck,