[Gaz64]

If you must have a fuel pressure gauge under the bonnet, a non-liquid filled version is way more accurate.
The liquid filled versions are not vented to atmosphere.
All bourdon tube mechanical gauges need to be vented otherwise they become inaccurate as the ambient temperature increases or decreases from the assembled temperature.

Set your fuel pressure at 6-7 psi engine cold, ambient of 25 degrees C.

If the gauge then experiences temps of 80-90c, the gauge may read near zero.

I would prefer to run a more expensive stepper motor electric in-cabin gauge 270 sweep with a rail mounted pressure sensor.