I've added a new feature to the firmware release 1.5.0 
- power offset 
.
Now you can add attenuators between the generator and the meter input and the values displayed are still correct.
When an attenuator value is set in the configuration, the unit (lower right corner) changes from dBm to dBO ;-) O like offset.
The following steps are available in the setup:
0dB,-3dB,-6dB,-10dB,-20dB,-30dB,-40dB,-50dB,-60dB
I've tested it with a 40dB power attenuator from Funkamateur, a 10dB attenuator from miniCircuits and my K3/100 and got the following data:
| Power (W) | dBmcalc | dBmmeas |
| 1 | 30,00 | 30,37 |
| 5 | 37,00 | 36,92 |
| 10 | 40,00 | 39,76 |
| 25 | 44,00 | 44,47 |
| 50 | 47,00 | 47,21 |
| 100 | 50,00 | 49,54 |
Now the question is, what are the correct values? Nevertheless the measurement results are looking pretty valid.
The latest code including my libraries can be found on GITHUB.