As you know there is snow humidity and air humidity, both are factors in wax selection in the race layer. I used the term humidity in a general sense and most junior (14-18yrs.) atheletes and their coaches around here do not distinguish between the two. And to keep things easy, I refrained from adding that factor into my wax mixes.
Around here it's easy to make a ski fast - our snow primarily comes from: manmade first, lake effect second, and Colorado lows last. All these snow types are high in humidity, and since we are surrounded by the Great Lakes, we are rarely below 50% air humidity regardless of the wind direction.
I could see that selecting the race layer can be very challenging for western speed events, where you can run into many variables in one run. Waxing for tech events is not as much rocket science though imo. If you want challenging wax choice scenarios - get a job as a tech on a national nordic team. Those guys have it tough. In Calgary '88, a Canadian team nordic tech got fired on the spot for a bad wax selection before the athelete even finished the race.