Calculate with lengths

Hi, I have the following items:

Number:Length CurrentRain “Current rain [%d mm]”
Number:Length RainToday “Sum of todays rain [%d mm]”

Now I would like to perform a calculation in the rules:

RainToday.postUpdate(CurrentRain.state as Numbers + RainToday.state as Numbers)

This calculation results in a faulty position after decimal point (0,001mm instead of 1,0mm).
What am I doing wrong?
Thank you!

Try something from this thread? You need to account for the units of measurement.

Thank you. Replacing

as Number


as QuantityType