Does anyone have rule of thumb kind of thing for the thickness of the backing board? I typically use Acid Free Artcare Backing Board in 3/16 inch thickness. Typically, a photograph will be T hinged on to it directly. Occasionally, I will dry mount my photograph to a 2 ply mounting board then that goes in front of the backing board. I understand thicker is better for stiffness and flatness but sometimes the moulding doesn't have enough space for 3/16 inch and I am forced to use thinner 1/8 inch kind. It seems to work OK and I've used 1/8" for 18x12 size. Is there any rules/suggestions for what thickness to use as minimum for various sizes? I use metal and wood moulding materials. All of my stuff comes from Framedestinaion, if you need to see exactly what kind of stuff I'm using.