Dual Gauge Leakdown Tester

I'm thinking of adding a dual gauge leakdown tester to the toolbox but have a few questions. I'm entertaining the idea of building my own rather than shelling out the dough to Summit or refinancing the house with Snap-On. My understanding of the tool is that it consists of a pressure regulator, pressure gauge, fixed orifice tube, another pressure gauge (in that order) and a line to the head. It works by measuring the pressure drop across the fixed orifice as shop air is lost (hopefully in very small amounts) at the rings/valves/HGs. A 100psi reading before the orifice and a 90psi reading afterward would yield a 10% leakdown. If this is true, wouldn't a change in the size of the orifice lead to much different readings from gauge to gauge? Is there an industry standard for the diameter of the hole? All that I've found puts the diameter at about .040 or 1mm. Does anyone know what size the big buck ones use?

Thanks for the help,
Anthony

btw:Manny keep the orifice comments to yourself!