aKula

Ok it's pretty simple:
For the case above you want the percentages to make 100 so:
87.5*X + 50 = 100
So X is (100-50)/87.5 = 0.5714
Then you use this X and multiply the rest of your probabilities and you'll get 100 as the sum of the probabilities, which is what you're looking for. (Alternativley you can express the probabilities in their "correct" form ie. X = (1-P)/Sum(Q) where P is the probability you set and Q is the sum of the other probabilities in your original set).

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