Very intersting topic, but has raised more questions than answers i.e.
I use a Deben scope for F.T. My mate uses a Tasco, when the temp.reaches around the 24 degrees C . my scope over ranges his under ranges, as far as I can see the main difference in operation is that when I paralex from my zero out to say 55yds.I turn the sidewheel towards me, with his scope it' the opposite.
So is it a function of temp. affecting the gearing ?.
Conversely I find that during the cold spells i.e. around 4 degrees and under my scope under ranges by an equivalent amount, 5 yards at 55yards.
Is the variance proportional to size of the objective lens,i.e. is the difference in perceived over actual greater for say a 50mm lens as against a 34mm lens.
Last point for now, how do our maksmen in Afganistan compensate for far higher temps. and ranges in excess of a mile?