I wouldn't call the boost gauge stupid if you don't know how to use it. The aem tru boost doesn't use "spring pressure", uses duty cycle in %.
So if you want to run 20psi and you set the A-setting for 20 IT WILL NOT give you 20psi. Each car is different so there's no right value for this but on my car to get it to about 20psi the duty cyle setting was set to 45.
If you want to run 25psi for example the duty cyle might have to be 70 or so. You have to play around with it and find out what works for you.