Measuring Incoming Gas Pressure to Stove
The standard gas pressure supplied to houses is 7" WC. Go to the website of your gas supplier and find out what they supply.

If you want to test this yourself, just connect the manometer to the tap and see what you have. This will be a static test and the pressure could drop when the gas is flowing, but probably it won't drop significantly. It is marginally better to use a Tee and test the pressure under flow, but for a minimal verification a static test is sufficient