So given the following info:
Aircraft at FL180
QNH = 1025mb
Using 27ft/mb (ISA lapse rate?) gives an altitude of 18,324ft AMSL.
Using 48ft/mb (Calculated lapse rate at 18000ft) gives an altitude of 18,576ft AMSL.
(FWIW, the term "lapse rate" is conventionally used to describe temperature variation with height, not pressure variation)
If you start at FL180 in an aircraft with 1013 set, and wind the subscale to 1025, you'll find the altimeter reads about 18,300 ft, not 18,600 ft. Why? Well it's true that levels separated by 1 mbar are about 48 ft apart. But you didn't make the aircraft "12 mbar higher", you changed the subscale by 12 mbar. That's like adding a 12 mbar column of air
at the bottom of the column on which the aircraft sits, and therefore the appropriate multiple is 27 ft/mbar.