You can probably get a good idea from this teleflex chart, but a lot depends on how you set the hoses up. If you "T" into the telltale hose with a 1/8" restriction, it will read differently than if you cut in at the exhaust housing.
The main benefit of a pressure gauge is that you get used to what it normally reads so that you have a heads-up when it is not functioning normally.
The only way to know what "normal" is would be to install a new impeller when you install the gauge. Take idle and wot readings and write them down somewhere. Any deviation from these numbers over time indicates a deteriorating impeller.
