What you could do to verify hhow your vacuum really is, just boil a know substance under reduced pressure, note the temp it comes over at.
Take a look at this post for a link to a vapor pressure applet that will tell you your real vacuum.
Post 522600 (https://www.thevespiary.org/talk/index.php?topic=10783.msg52260000#msg52260000)
(OcoteaCymbarum: "Online vapor pressure applet", Newbee Forum)
Then you could figure out how much you can really pull, as it never gets(IMHO) to what the manufacturer's claims in most bees labs.