The turbo boost gauge depends on information being correctly entered into the vehicle profile for some types of vehicle.
The most important fields to fill in are Engine displacement and Fuel type.
There are options in the Units settings to control the behaviour of this sensor - you can for example disable negative readings switching to in/Hg units.
For petrol vehicles, at idle, your vehicle will always measure vacuum (or negative psi if you have configured this behaviour in the settings)
For diesel vehicles (which do not use throttle to control engine speed/power) this reading should read on or very near 0 when at idle. You may see a slight vacuum due to a small flow restriction created by the air filter. If you see a large vacuum then check you have setup the vehicle profile correctly (and told the app to use that vehicle profile)