For me personally, if you just consider that the altimeter always knows the correct outdoor pressure it all becomes a bit easier.
Say the pressure outside is 1010 - the altimeter knows this. All it doesn't know is what the sea level pressure is. Why would it, unless it's by the sea.
Once it finds out, it can calculate your altitude accordingly using the 30ft per hPa rule. Which is why the lovely people in ATC give you this in the form of the QNH.
If you tell it the sea level pressure is 1013, it will think it is 90 ft above the sea.
If you tell it the sea level pressure is 1014, it thinks it's 120ft above the sea and so on. Thus, as you turn the scale up, the indicated altitude will go up.
