Aftermarket boost gauges come stock with a vacuum line and a pressure sensor mechanism. When turbo boost pressurises the vacuum lines, the boost gauge's needle is preset to display in a numerical value the current pressurisation of the boost gauge's vacuum line. As such, boost gauges require no user calibration. If your boost gauge is displaying the incorrect boost value for your turbo system, it could be due to multiple causes.
Find out what the stock boost pressure is for your vehicle, often listed on vehicle spec lists. This will allow you to know if your boost gauge reading is off by comparing it to the stock number.
Accelerate your vehicle at wide open throttle and note the peak boost pressure held across the RPM band as given by the boost gauge. Compare this number to the stock number to see if your boost gauge is off calibration.
Splice the boost gauge vacuum line's T-fitting into a new vacuum source if your boost gauge is giving an inaccurate boost reading. Some vacuum lines may be inadequately pressurised, thus giving the boost gauge an inaccurate turbo boost reading. Try using the vacuum line connected to the engine's intake manifold for your T-fitting vacuum source, as these generally have accurate boost pressurisation. Replace the old vacuum line from which you removed the T-fitting.
Repeat Step 2 with the boost gauge T-fitting connected to the new vacuum source. If the boost gauge still gives an inaccurate pressure reading, the gauge itself may be faulty and in need of replacement. Alternatively, turbo system problems such as boost leaks will affect the boost pressure, and thus the boost gauge's readout, so ensure your turbo system is free of malfunctions.