@eckhard: the 4% formula is the "simple" formula but limited to temperatures down to -15C. Beyond that, PANS OPS offers a table that covers lower temperatures but is limited to 5000ft and two more formulae, the more complicated of which was the reason for my post.
As probably all those formulae are but child's play for a modern Computer, IMHO it does not make sense to use the simplified versions rather than going for the most accurate calculation.
@WeeJam: I have understood lapse rate (which is 1,9812C per 1000ft to be exact
) but lapse rate is just one building block for calculating cold temperature correction.
Any other input welcome. It must be possible to solve this!