Oil pressure?

[Niall]

Found a few threads but none that really have my answer.
What sort of oil pressure should i be expecting? Im getting about 80psi on idle (cold) dropping to about 40 when the engines warm (although oil wasn't. Ive literally just run it up on the drive).
Also picks up with rpm. Ive heard a general rule is that you should get 10psi for every 1000 rpm but not sure how true this is?

Cheers.