Not sure what time his normal bedtime is...but what DH and I always say is (as an example) aiming for 630 bed, but willing to do anytime between 600-700 as needed. So meaning, if at 545 he is melting down and yawning, all things will be prepared to scoop him up from the dinner table, take him to his room, PJs on and into bed at 600 if that is what needs to happen. Otherwise he will be ready to go into bed at 630, but if things are going well and he is enjoying stories and does not seem tired and that we will end up with a UT-causing-OT situation, then we will read /watch cartoons until closer to 7pm.
So totally not helpful I am sure, but I do find that we just need to be prepared with a "window" and be ready to work within it as needed. I would say expect 45-60 minutes before his normal bedtime and then judge from there on how things are going. We skip baths on these nights, no matter how dirty he is (!), because they just have potential to throw so much off kilter!