Tire spacing dually

[Cppr Busa]

I put 265-75-16 bfg all terrains on my dually and I made 1/4 spacers for the rear. I didn't know how thick it needed to be, so with some searching on the net I decided to try 1/4 thick spacers. I got them all bolted up and with the 1/4 spacer it has about a 1/4 gap on top and they just touch on the bottom.

So my question is how much space do I need between the tires? I don't tow hardly at all. Thanks for any and all help