This reading starts the when the key is turned and it measures the pressure in the plenum. 100% is atmospheric at any altitude. It is measured after the SC. 102% sounds perfectly normal. The engine idling with the throttle plates closed will give you a bit of boost. You can get a reading below 100% by opening the throttles at low rpm and the engine can ask for more air than the SC can provide at a low RPM. In this case the engine is hesitating, kinda like a car with the accelerator pump that is broken. At above say 2k it is pretty much impossible.