First of all: I do not own a calibrated generator in the frequency range of the AD8318 chip. The closest approximation is a Kenwood handheld that emits about 50 mW at 450 MHz. That is my source.
With 30 dB attenuation I could bring that signal down to about -13 dBm that the AD8318 reads as 0,904V, which is in the ballpark of -16 dBm according to figure 4 of the datasheet.
What I could confirm is the -0.025 V/dB slope by adding and removing attenuators. This let me compute the 0V intercept, but the Arduino firmware does something weird and tells me I am feeding a much higher power.
There also is a dependency on impedance matching at the AD8318 board input, since swapping my attenuation chain changes the detected value, appreciably.
The NanoVNA did help to confirm my attenuation chain value and impedance mismatch.
For the time being, since I lack too much instrumentation to turn the circuit into something precise, I will use its output voltage to peak an RF signal and I will not care of its actual Wattage (trimming 10 GHz filters on PLL modules).
