300ft AMSL means 300ft above metric sea level.
Metric sea level is a constant.
Elevation is a constant.
The height of the mast above ground is constant.
The mast will always be 300ft above metric sea level.
The atmospheric pressure may change every day, the only effect this will have will be to change the QNH....i.e. changing the sub scale you have to dial into the altimeter to get the correct readings.
Altitude is relative to MSL. MSL is constant. Pressure changes. So you have to change the sub scale to 'restore' the correct relationship between altitude and MSL.