The electrical over-boost protection circuit will cut the fuel when boost pressure reaches 1.1 BAR (16.1 psi) on the 617 engine.

I have also installed a mechanical 'blow-off' (safety) valve, like the one used on air compressors, to 'pop-off' at 15 psi, just in case the factory system failed.

I have found that welding gauges, particularly the one used for Acyetlene, are not only highly accurate, but also have a 'redline' beginning at 15 psi. This is what I used in the dealership to check/correct boost pressure on customer's cars.

The 617.950 Shop manual recommends removing the air filter element and 'dragging' the brakes to keep engine rpm at 4000 in second to check boost pressure.

I leave the air filter in place and check pressure at wide-open throttle in second gear, with the engine against the governor at about 5000-5200 rpm.

I used to set every 617 I worked on (during a major tune) to 12-14 psi of boost pressure.

On the Garret/Air Research Turbos with the internal wastegate adjustment, 9mm of exposed threads above the diaphragm piston will yield 13.8 psi on a good running engine with a defect-free turbocharger.

Best of luck on your projects...Bert