It's strange though
Don't go wavering now, Dave, you've got it!
Pressure altitude is,
by definition, what the altimeter reads when 1013mb is set on the subscale.
And we're told the altimeter reads zero when the subscale is set to 992mb (QFE,
by definition).
Thereafter, the maths couldn't be simpler! You move the subscale from 992 to 1013 (i.e. by 21mb) and the altimeter display changes from zero to 630ft (i.e. by 21 x 30). The altimeter has now changed from showing height above aerodrome elevation (i.e. 0ft) to the aerodrome pressure altitude (i.e. 630ft). QED.
Don't be misled by the questioner, bose-x or friendly flier, your understanding is now fine. Good luck with the exams.