Dear Michalis Angeli,
In the Wiring layout you should define the average length for one string (blue) and the distance to the main box to the inverter (green). If you have multiple strings and/or inverters this will be reflected in the Global array and the Field global wiring resistance. See example below. It is this number that will be used in the simulation.
If you wish to simulate a more pessimistic scenario, to be on the safe side, you can artificially increase the Loss fraction at STC to, for instance 0.8%, compared to the calculated values of 0.65 from the example.