BB
Q1) Think of it as 30ft =1mb rather than 1mb = 30ft. This approximation is used when working out the altitudes & heights of aircraft and /or airfields when low level since any aircraft flying up high is likely to be flying a FL using 1013mb. For example if you know the QFE on the airfield and the elevation of the airfield you can work out the QNH (roughly).
Eg. QFE = 998mb Elev = 300ft using 1mb = 30ft
then QNH = (300/30) + 998 = 1018mb
This can be used in reverse.
Q2) As said before.