Correct! Everyone in Europe flies on QNH (except US military and some US leased military aircraft). We don't know "alimeter settings" here.
So aerodome A is 800 ft AMSL and aerodrome B is 500 ft AMSL.
They assume 1 MB = 30 ft (That is one like QNH 1001, 1002) in ISA conditions.
Difference...