Hi Michael,
I am using the same hardware from MDT. You can also use the beautiful widget from Nico. There are minor modification needed to run it with 1bit element, but you can use the code, that I already posted. If possible could you try whether you have the same issue with rounding?
https://community.openhab.org/t/oh3-heating-widget-based-on-css/115107/58?u=snowy