I added customizable variance to my program and ran it from +- 1 through +- 10 (e.g. stack size / 2 +- 1, etc.). The results are unexpected.
Variance: 1
Permutations: 8
Yang (old) percent: 25.000000
Yang (young) percent: 25.000000
Yin (old) percent: 0.000000
Yin (young) percent: 50.000000
Variance: 2
Permutations: 64
Yang (old) percent: 18.750000
Yang (young) percent: 31.250000
Yin (old) percent: 6.250000
Yin (young) percent: 43.750000
Variance: 3
Permutations: 216
Yang (old) percent: 16.666667
Yang (young) percent: 33.333333
Yin (old) percent: 8.333333
Yin (young) percent: 41.666667
Variance: 4
Permutations: 512
Yang (old) percent: 18.750000
Yang (young) percent: 31.250000
Yin (old) percent: 6.250000
Yin (young) percent: 43.750000
Variance: 5
Permutations: 1000
Yang (old) percent: 20.000000
Yang (young) percent: 30.000000
Yin (old) percent: 5.000000
Yin (young) percent: 45.000000
Variance: 6
Permutations: 1728
Yang (old) percent: 18.750000
Yang (young) percent: 31.250000
Yin (old) percent: 6.250000
Yin (young) percent: 43.750000
Variance: 7
Permutations: 2744
Yang (old) percent: 17.857143
Yang (young) percent: 32.142857
Yin (old) percent: 7.142857
Yin (young) percent: 42.857143
Variance: 8
Permutations: 4096
Yang (old) percent: 18.750000
Yang (young) percent: 31.250000
Yin (old) percent: 6.250000
Yin (young) percent: 43.750000
Variance: 9
Permutations: 5832
Yang (old) percent: 19.444444
Yang (young) percent: 30.555556
Yin (old) percent: 5.555556
Yin (young) percent: 44.444444
Variance: 10
Permutations: 8000
Yang (old) percent: 18.750000
Yang (young) percent: 31.250000
Yin (old) percent: 6.250000
Yin (young) percent: 43.750000
  Here is the average percentage if the split variance is between 1-5 stalks:   Yang (old) percent: 19.834
Yang (young) percent: 30.166
Yin (old) percent: 5.166
Yin (young) percent: 44.834   And here is the average percentage if the split is between 1-10 stalks:   Yang (old) percent: 19.272
Yang (young) percent: 30.728
Yin (old) percent: 5.728
Yin (young) percent: 44.272   I think these last numbers are the most accurate for real world use. 
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