A good discussion on this topic is Thread798-51260, or get a copy of the following thechnical paper on choked flow thru square edged orifices:
author A.J. Ward-Smith , Int J Heat Fluid V1 no 3 pp123-132 1979.
I think Dennis was the originator of thread798-51260. Consequently, I think he is on top of this subject and is following up with what I think are results of his findings on the subject.