I think your voltage values are then between 3V and 2.1V…
But i read my voltage value from HmIp SWDOs (Fensterkontakt), i currently get between 1.1V - 1.3V ?
My Heating Sensors (EVO) or my wall-thermostats (Heizkörperthermostate), Buttons (Taster) are also in range 2.1 - 3V.
You also have some window sensors there… do u know why is it like this showing 1.1V ?
Edit: Ok, that was my bad 
1 Battery vs. 2 Batteries
But now i have another question…
So the scale with 2 batteries is between 3V - 2.1V. (where 2.1V means 0%):
3V 100%
2.9V 89%
2.8V 78%
2.7V 67%
2.6V 56%
2.5V 44%
2.4V 33% —> default LOW BATTERY Threshold (in CCU)
2.3V 22%
2.2V 11%
2.1V 0%
I cannot convert your formula for 1 Battery (1.5V)
1.5V 100%
1.45V 89%
1.4V 78%
1.35V 67%
1.30V 56%
1.25V 44%
1.20V 33% (default LOW BATTERY Threshold for 2 Batteries)
1.15V 22%
1.10V 11% —> default LOW BATTERY Threshold 1.1V
1.05V 0%
Now I have a SWDO, which shows me 1V and Low Battery… it does not match the above scale…
maybe it is like this?:
1.5V 100%
1.4V 83%
1.3V 67%
1.2V 50%
1.1V 33%
1.0V 17%
0.9V 0%
So the formula is: ((voltage - 0.9) / (1.5 - 0.9) * 100) ?
Do you also use like this?