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You're right. I posted with an old formula.
Here the correct one:
[display reputation] = max( log10(abs(reputation)) - 9, 0) * ((reputation >= 0) ? 1 : -1) * 9 + 25
I updated the post

The abs() is because a logarithm from a negative number is not possible, so they have to make it positive before the logarithm. No need to worry :)