@ Dr. Recommended :
The larger N is, the more (N-2)/2 tends to be equal to N/2.
In other words, with the scoring system that you propose, there is not much difference between the points awarded for a solo and the points awarded for a 2-way draw.
For example, a 2-way draw in the variant Fall of the American Empire IV (which includes 10 players) would reward 4
to each of the 2 drawing players, while a solo in that variant would reward only 5
!
I know that 2-way draws are not frequent, especially in Gunboat games but the same reasoning can be made with a 3-way draw : the larger N is, the more (N-3)/3 tends to be equal to 1/3, etc.
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Let me please rephrase it :
With the scoring system that you propose,
• in each game drawn, a total of N-D points is awarded ;
• in each soloed game, a total of N/2
is awarded, which is always less that N-D (in the case of a solo, D=1), as soon as N>2 (which is always the case in a Diplomacy game).
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As a conclusion, the proposed scoring system does not reward enough solos.
May I please suggest, as a solution, that the scoring system should simply always reward (N-D)/D to every player that finishes the game without being eliminated, which would reward N-1
for a solo.
Another possibility is to encourage even more solos by giving N points to the winning player in case of a solo.