[werk_ag]

Don't touch the pot for this.
How long are the cables between the Solar / UV interface and the TX unit? If possible use twisted par cables, and keep them as short as possible.

Usually readings in the dark are bellow 4, however for some reason they are above that value.
Search for this code on the RX software:

Code:

      if (RX_Data[2] < 4) RX_Data[2] = 0;        SolarRad = map(RX_Data[2], 0, 1023, 0, 1400);    // Outputs an integer, between 0 and 1400 W/m2

change to this

Code:

      if (RX_Data[2] <= 6) RX_Data[2] = 0;        SolarRad = map(RX_Data[2], 0, 1023, 0, 1400);    // Outputs an integer, between 0 and 1400 W/m2

For a final calibration, wait for a clear sky day, and try to compare your readings with a nearby trustable station.
If they don't differ too much, try to increase or decrease the 1400 value on the code above. You really need to do this in a clear sky day.