We do 18" grid, 12" deep where possible, and holes are the diameter of the airspade tubing. 2"-ish.
I don't know the proportion. 2 shovel scoops mulch, 1 small scoop worm castings, 2 small scoops perlite :). I'd have to get out a measuring cup to see what it comes out to.
It's not mixed with...