That's pretty good and close enough. Who's to say your 28psi reading isn't actually 30? Can't really trust gauges or even testing equipment to be 100% accurate.
As Jerry has said before, as long as oil is being replaced to the sump at exactly the same rate that it's being applied to a particular location in the engine, Zero oil pressure would get the job done without harming the engine. So 28psi instead of 30 isn't anything to worry about.
