Compute mean gas velocity in a tube given flow rate and orifice diameter...

Pressure/vacuum computations are ideal applications of Bernoulli's Law determined solely by gas velocity and density, and do not take into account pipe length or roughness.

The Reynold's number is pretty big isn't it? This means we
have turbulent flow and that surface roughness in your pipe is
__very__ important and anything you can do to either reduce Re
or smooth the pipe and joints is a good thing. (Small Re < 2,000
indicates that gas viscosity overwhelms inertia and thus flow is
laminar; large Re > 10,000 means inertia is the overriding factor
in flow and thus flow becomes turbulent.)

My nominal atmosphere numbers are 101.325 kPa pressure and 1.225 kg/m^3 at 288.15 K.

Corky Bell says 300 fps max through the TB of a turbo motor...

Use the MM function, as in "MM(40)", to enter diameter values in millimeters. You can enter expressions with arithmetic operators into the fields, they will be evaluated properly. For example, enter "1.5+MM(20)" into diameter meaning 1.5 in + 20 mm.