I assume you’ve got a modern plastic tank. In which case it’s not easy to work out use rate in litres/day. The old metal tanks were much easier, just measure it and you get litres/mm, which is the same over the full tank. Moulded tanks have a different cross sectional area as the height changes.
If you know what the flow rate is of the burner, the jet diameter (it should be in the boiler spec) and you measure how long the burner is on for (I did this with an Arduino which detected the sound of the burner starting) you can get an estimate of the volume per mm drop of the sight glass. For my 1000l tank, it’s about 1l/mm in the middle (200 to 800 litres.
Also make a note of the reading just before it gets filled and just after, the difference is the amount added.
But it’s not easy getting a reading in litres!