Yep, I think you are right.
When coding HEX values to change voltage I am pretty sure it should be xx,1x for percentage of maximum (relative) and xx,0x for voltage values (absolute) in the NETTODAT file. I never heard of anything different.
In NETTODAT it reads for example 8D,0C
The first character of the second byte (=0) indicates this value is written in voltage rather than in percentage.
To decode the voltage simply put the second character of the second byte (=C) in front of the first byte and calculate
Ueff * 4095 digits / 18V = [C8D in dez]
I think you can calculate the rest for your own.