Hi there!
We just spoke on the phone, but I'm answering your question here anyway. To others can learn from it as well

The correct universe to output is whichever you configure the enttec octo, in this case, to receive. There is no 'rule' in that sense.
Remember that Art-Net is '0-based' (zero). So 0.0 is the first universe in the first 'net', 0.1 the second up to 0.15 where you go to the next net: 1.0, 1.1, 1.2... 1.15. Then 2.0,etc.. All the way up to 15.15 being the very last 256th universe (16 universes on 16 nets).
In some interfaces, universe 1.0, for instance, might also be indicated as '17', In those cases you'll have to calculate the correct one for your setup.
Cheers!