As described here, I've build an RF Powermeter.
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.