Try online calculators? Do you really want the maths behind the ISA figures?
http://www.onlineconversion.com/pressure.htm tells us:
30 inch of mercury = 1 015.92 hectopascal
1 000 hectopascal = 29.53 inch of mercury
30 inch of mercury = 14.73 pound/square inch
1 000 hectopascal = 14.50 pound/square inch
http://www.digitaldutch.com/atmoscalc tells us:
minus 300ft ISA conditions = 1024.28 hectopascal
= 30.25 inch of mercury
=14.86 pound/square inch
So if the a/c is on the ground and the barometer is 30inches the differential pressure is 0.15 lb/square inch.
Now if the a/c is at 20000ft ISA, digitaldutch gives atmos pressure as 6.75lb/sqare inch but you tell us it is 7 pound/square inch in the cabin. So the differential pressure is 0.25pound/square inch.
If you wish to know more about density altitude rather than pressure altitude try
http://wahiduddin.net/calc and the discussions he includes. From memory he supplies all the underlying maths for pressure and density altitudes and is a fun guy to boot.
I hope this helps
Best Rgds
The "E"