I usually put down 3" of cover when mulching/decorating.
Take your square footage, in this case 40 and multiply by .25, this represents 3" of thickness which will give you the volume required in cubic feet. In this case it's 10 cubic feet. Now divide this by 27, the number of cubic feet in a cubic yard and you've converted your volume to cubic yards. In this case it's .37 cubic yards.
Most clear stone weighs in at 1 1/4 tons per yard, so we take the .37 and multiply it by 1.25 and we get .46 ton, or damn close to the 1/2 ton that cutntrim recommends.