no boost is just a general term for possitive pressure. be it psi/bar/kpa. as soon as the car, regardless of rpm went into possitive pressure (0.1bar for example) you would be "on boost". if your maximum level of pressure produced by the turbo is 0.8 bar. your car would be running "0.8 bar of boost" or "boosting at 0.8bar" etc. get it?