For a quick practical check of accumulator setting.
1. Taps closed, pump on.
2. pump off
3. open a tap and collect the water in a bowl until it stop flowing.
4. measure collected water.
If you get close to half the volume of your accumulator, all is well. (ie for a 5 litre accu about 2 litres collected is fine)
If you get a lot less than half, then let some air out of the accumulator. If you get much more than half put some more air in.
You don't really need a pressure gauge.
edit to add - if no water comes out, there may also be no air at all in the accumulator, check the valve to see that you can release some air.