Dean, thanks,
I dont really understand your first question. If you are asking the question I think you are, then my answer is that they are each filled with 70lbs of water, and with the added rear weight of the backhoe the ground pressure is very much higher than 0 psi.
They are only 12" wide each, with the duals I used before I had 16" of ground contact on either side.
I didn't fancy the idea of the ether in case I put too much in, and caused it to blow up too much, therefore damaging my rim, or the very expensive tyres!
Cheers