Altimeter Setting
Axel-Flo, I think what you originally wanted to know was where it was written down. Fascinating subsequent posts but very little to the point.
My company also had a "within 2000 ft" SOP until a few years ago when, like yours it changed to "when cleared". I can't find exactly what you're after, but offer a couple of thoughts.
ENR 1-7-2, 5.1.4 uses the term "when cleared" to FL to say that from then on "vertical posn will be expressed in terms of FL", in other words when cleared...set 1013.
Also, such procedures are set by Ops Manuals, which usually evolve from liaison meetings between operators and the CAA (among other things) and this one has the stamp of that kind of process about it.