Notice how the each of the other probabilities have decreased by the same amount (9.375)? Now this is the case whenever you did such a step (all the other probabilities decrease by the same amount) you could use that to make an easy way of calculating it.
So just find by how much the one probability has increased:
50-12.5 = 37.5. Now divide this by 4 to get 9.375 and subtract this from each of the probabilities.
Let P be the probability you set, Po be the value this probability was beforehand.
Q is any of your new probabilities that are changed due to the change in Po. Qo is what Q was originally.
Q = Qo - (P - Po)/(n-1)
I guess you could use a similar method if you wanted each of your probabilities to decrease by the same percentage of their original value.
Edit: Of course this may be exactly what you were doing and were just after a faster method, if so ignore the above post
