My days with DS are v similar although. I dont have any older DCs so don't have that excuse for the chaos! He usually only naps for 45 minutes tops at any time in the day, so if he wakes at 0550am one day (booooo!) and 0715 the next day (yay!!!) it throws out his whole routine by over an hour, and nap failures/randome long naps when I'm lucky also have a effect.
As he is creeping towards 6 months I am try as a general rule not to let him start a nap after 5pm...and even then I would only let him have 30 mins as would start clattering around at 5.30pm...basically cos I want him in bed by 7.30pm latest!!
So if we have a day when he wakes up from a nap at 3pm I would try to put him down for a quick nap at 5pm. If he wakes up any time after 3pm, I would try to keep him up till bedtime, but would bring bedtime right forward and put him to bed at 1815 if necessary - doesn't seem to affect his wake p time in the morning.
But this only works if he has had a good hour's sleep for the nap which finished just after 3. If it was only a 30 minute nap, I would probably try putting him down at 1630, as the length of time Es capable of staying awake seems directly related to how long the nap was, if that makes sense?
Basically OP, the answer is: yes, our days are unpredictable chaos and the naps vary wildly in their time/duration every day! The only thing that's certain is is goes down 2 hours after he first wakes...after that it's all crazy!