If you expect to set tasks at about 200 km, what should you set
the Nominal Distance at?

Given the Dmax = 200 km, Dmin = 5 km, Gnom = 20 the line above gives the
relationship between the meet director's choice of nominal distance and the
average distance required for full validity.

Given the Dmax = 200 km, Dmin = 5 km, Dnom = 120 km, the chart below gives the
relationship between the meet director's choice of percentage of pilots at goal
and the required average distance flown for full validity:

Full Distance Validity also depends on what percentage of the pilots fly further
than the minimum distance. Given the Dmax = 200 km, Dmin = 5 km, Dnom = 120 km,
and Gnom = 20, we get the following relationship:

Looking at the 2013 Worlds:

If the nominal distance had instead of being set to 80 km had been set to 120
km, the Distance Validity would have still been at least equal to 1 each day,
despite the fact that day five and six were stopped. So nether of us would have
set the Nominal Distance long enough to differentiate between days where we went
below average and those where we were above average.

If we had set the nominal distance to 180 km, we would have had an average flown
distance of those flying further than the minimum distance of 130 km for the
Distance validity to equal 1. We would have noted before the first task that we
were probably setting the first task (and task 2, 3, and 5) at too short a
distance. In addition, the second, third, fifth and sixth days would have been
devalued. The fifth and sixth days because the task was stopped. The second and
third because the tasks were called too short.

It was only after the fifth task that we started calling tasks long enough
to get pilots to fly far enough to be completely valid assuming that pilots
could in fact on average fly 180 km on an average day.

If we had set the nominal distance value equal to 180 km then these would be the
Distance Validity values for each of the days:

The two days that were stopped are devalued the most, which is what we would
hope for.

This would have been the results if we had chosen that 180 km as the Nominal
Distance:

#	Name
1	Manfred Ruhmer	8505
2	Alessandro Ploner	8389
3	Filippo Oppici	8090
4	Attila Bertok	8073
5	Primoz Gricar	7867
6	Grant Crossingham	7819
7	Pedro Luis Garcia Morelli	7785
8	Balazs Ujhelyi	7696
9	Scott Barrett	7694
10	Antoine Boisselier	7666
11	Jonny Durand	7615
12	Robin Hamilton	7604
13	Paris Williams	7515
14	Gerd Dönhuber	7456
15	Michael Friesenbichler	7440
16	Gordon Rigg	7402
17	Christian Ciech	7375
18	Gary Wirdnam	7306
19	Carl Wallbank	7172
20	Rohan Holtkamp	7169

You can find the actual results
here or here.