Ah, interesting! You’re not tempted to, say, slack off right after a ping when you know you have a 5-minute window of safety?
If you wanted to plug that loophole: Official Reference Implementation of the TagTime Universal Ping Schedule
Either way, I think it’s better (or no worse? I’m not sure) to estimate time spent as simply 45 minutes (or 50 in your case) times the number of pings you got, without caring about when the day started or how many total pings happened.