When I picked up the trailer, the dealer noted where the lower support bars were inserted. He said the way they determine it is by using the highest slot, then pull the bed out and if there is any resistance, drop down to the next slot, etc.
He said the when the trailer is assembled there is a variance plus the support rods can have a small variance too.
For me it was the center for the front, and the second from the bottom on the rear.
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!