Me, I just "eyeball" the ride height but... Technically, the 'best' way is...
Let all the air out of the bags
With average load in the back measure the height front and rear of the vehicle (yes, front as well) use, say, the Bullbar and rear bumper, towbar, whatever to give you a ride height relationship of your particular suspension setup and lift etc. Remember that the vehicle is designed by Nissan to have some load in the back plus fuel etc and not be dead empty so if it is 'empty' maybe take a couple of mils off the measurement.
Put a couple of PSI in the bags. This is to inflate the bag and prevent possible pinching more than actually adjust the levelling the vehicle.
If the bum is dragging a bit due to draws or whatever you may need an extra couple of PSI (I run mine at 8 PSI due to a lot of accessories permenantly mounted in the back)
Whenever a large load is added (Cargo, Trailer, whatever) and the rear sags you should add air pressure until the vehicle ride height relationship is back to the measurement you got initially IE back is 'X' mm higher than front.
Now that sounds long and involved but it only takes a few minutes really and you only have to do it once to suss out for example
"Van on and back loaded for trip = 20 PSi,"
"Roof rack loaded and dual rear wheel carrier with both spares on = 15 PSI"
As I said, the other method is just wack in some Air till it looks right to the old Mark 1 eyeball.
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