Leak-Down Testing

["Tony Clark" <lotus.tony@airmail.net>]

Can someone do a good job of explaining exactly how the leak-down
instrument works?
I've seen a diagram, I see that the air supply pressure bleeds
through a small orifice on the way to the engine cylinder and that
the pressure of the "supply" is compared to the pressure of the
cylinder to arrive at the percent of leakdown but:  won't the orifice
size affect the amount of air available to "leak"?  Is it just
arbitrary?  Someone clear me up on this, please!
Tony