By whatever method you make a vacuum source -- foot pumped, box pump
or vacuum cleaner -- a simple adjustable port will not keep vacuum even
with varying demand. To achieve this, you need some sort of reference
such as a spring or reference vacuum on a diaphragm regulator.
I like the diaphragm regulator because it is easy to build and adjust.
As long as the volume of the reference vacuum is kept small, a single
stage regulator is quite stable. For absolute precision you need to
regulate the reference vacuum with a primary regulator, not just a
needle valve or reference orifice.
This all sounds very complicated but I have a little box that is about
4" by 1-1/2" by 1-1/2" that regulates vacuum from one inch of water
gage to 150 inches, reliably and easily. I have not measured the flow
range for accurate regulation but it is more than would be needed for
just about any testing need you might encounter.