The basic firmware is now running. So I started analysing the accuracy of the used meter head.
First I made a simple value table showing the ADC values over power input: (Y Axis must be inverted!)
Applying a linear formula shows slight deviations.
A third order polynom show a mucht better match
All images are screenshots from Ulrich A. Maassen freeware application Polynom Maker Univariante Regression. A similar online application can be found here. The online version shows the coefficients in a much higher resolution.