How hot are you heating the tank up to? How many kWh are you putting in each night?
Maybe you can use both pockets? Bottom one for heating up the cylinder, top one providing extra info about what’s in the tank. Maybe by analysing both you can decide when to add more heat?
You would need a flow meter on the outlet from (or inlet into) the cylinder to measure exactly how much hot water has been used. Probably best asking how in a new thread.
A cheaper, less invasive, less accurate option might to strap a thermal or acoustic sensor to the pipe to detect when hot water is flowing.
Edit: see this topic for further discussion: Hot water tank temperature monitoring