Had a play around with the pump today. Works really well, I set one of the memories for 36 psi and used it on the bike.
It stopped at 36.5 and when I checked with my manual very accurate dial gauge it showed 37psi so I am happy with that. I guess it allows 0.5 psi for when you remove the valve but it doesn't seem to lose anything when you unscrew it.
A lot easier than messing around with a foot pump.