It's going to be ball park at best because there's no information for session length, longer weekend sessions sound like a reasonable thought. If that's the case though then the DAU count would be lower; fewer sweats playing for longer.
The way I'm working it out is sampling CCU for every hour over a complete day and using a session range with an average.
Though the number isn't going to be spot on, I figure as long as the formula is consistent it will show accurate drops by % over time and perhaps give more of an idea than the standard SteamDB DAU chart.