BadPostCount = TotalTPCount * TotalCheckPosts / (TotalTPCount + TotalFPCount);
Specificity = (BadPostCount - FPCount * CaughtCount / (TPCount + FPCount)) / (TotalCheckPosts - BadPostCount);

public float Specificity
{
    get
    {
        // Loving formulated by Jan Dvorak (stackoverflow.com/users/499214/jan-dvorak).

        var badPostCount = GlobalInfo.Stats.TotalTPCount * GlobalInfo.Stats.TotalCheckedPosts / (GlobalInfo.Stats.TotalTPCount + GlobalInfo.Stats.TotalFPCount);

        return (badPostCount - FPCount * CaughtCount / (TPCount + FPCount)) / (GlobalInfo.Stats.TotalCheckedPosts - badPostCount);
    }
}