the answer does not lie in whether you get the same density onyour radigraph as the astm ref.
The ref. is merely a size of discontinuity type reference at a specified sensitivety, IE 2-2t.
You could use a different film and the density will vastly different but the flaw still be the same size.
Remember that a radigraph is a 2d pic of a 3d object, andthe lost dimension is depth.
Hope that helps...