Calc with virtual sensors

[Hathor27]

Hi there,
I'd like to build a virtual sensor out of values of two existing sensors, let's say I have

• - sol1[2] as evapotranspiration value [mm]

• - sol8[2] as equivalent value for rain [mm]

Now I'd like to calculate the difference between both values and fill its result to an new virtual sensor sol9 - how can I solve that?

Would be great to read your ideas