Measure from the centre of the head to each side. Say that your 36' head has 19' to the right and 17' to the left, your entered offset would be 1/2 of 1/2 your difference. In this case, your offset is 6". This is a starting number and if it is crowding should increase by 1" and try it. .
I would measure to find the center of the header and line the gps antenna up with that and have no offset. Not sure what kind of gps you are running, but on my S2 outback it seemed like half the time it did not recognize the offset even though it was entered in there properly. If I would re enter it all was good until it was shut down and restarted, then start the process over again.