Thanks! So I was doing it backwards. 
If I may: Perhaps it would be less ambiguous phrased that way (if I got that correctly):
In the rare occurrence where a drift of the colors is noticed with “Observer 2°” – generally due to the conversion matrix –, “Observer 10°” must be selected instead.
But I’m not a native speaker either, so the only thing I can vouch for with any degree of certainty is the replacement of hyphens with en dashes. 