I'm curious do most of the passenger jets currently used around the world, have metric function? Especially metric alt indication on standby altimeter?
If the answer is "yes", then why do you do all those calculations (*3 +- pressure correction/field elevation)? Why not to set QFE pressure (QNH - field elevation in mb/in Hg) on standby altimeter and get the required value directly (metric alt)? Then all you need is correction for your speed (1000 meters or 10nm for 280 kt) and for the wind...