http://www.sensorsone.co.uk/altitude...onversion.html
Your first question appears to involve an aircraft sitting on the ground, pressurized, with a cabin pressure of -300'. If this is the case, and you're asking what the differential pressure is, then you'll need to know what the outside pressure is.
If you are to assume a standard atmosphere outside the aircraft and the internal cabin pressure altimeter referenced to 29.92" (or 1013 mb, as your pleasure takes), then the differential pressure is .16 psid, reference the chart cited above for standard atmosphere.
With a cabin pressure of 7 psi, you're holding a cabin of approximately 17,500', again reference the chart cited above. This, regardless of the actual aircraft altitude. Your differential pressure at is approximately .7 psid.
If instead your cabin differential is 7 psid, then your cabin at 20,000' is going to be approximately 1,800'.
Someone correct those figures if you will; I'm listening to four yelling kids and doing it in my head....far from exact.