I have a slow leak on car tyre, using a cheap tyre pump, it was telling me 50psi, when really it is 40 psi using the digital gauge.
Swaps to presta valves as well.
Well worth the investment.
while on the whole its ok, the rubber seal is not really up to the job, unless you hold the gauge perfectly straight it leaks and i dont think it would hold 300 psi, also the little spring in the connecter makes more problems than it solves, when ive got time i will look for a better rubber seal and remove the spring, then i would hope this would solve the things wrong with a good idea.