It sounds like you are on the path. We've always had it run in phases of different lengths.

For the fixed playtime, it's simple. Bedtime at 9, playtime at 8:30. If he's lost 5 minutes of playtime, then 8:35 playtime and 9 bedtime.

Actually, I'm using present tense above, but it's very rare anymore that he needs a small shock like losing playtime to get his attention. It seems like they need to understand how the whole ball of wax works together (ethics, morality) before they can use it (almost a perfectionism thingy.) Typically, we only have to provide specific feedback tied to some ethic, and he'll adjust his behavior. If it was something he knew better, he'll be more upset with his own self than anything I could say or do.