The so-obvious-you-d-think-it'd-be-in-the-library value converter used in binding a control to something-a-specific-bit-less-than-the-size-of-its-parent.

gistfile1.cs

public class SubtractConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        return (double)value - System.Convert.ToDouble(parameter);
    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        return (double)value + System.Convert.ToDouble(parameter);
    }
}