I got round this by adding an `IValueConverter` to the `Value` binding:

    Value="{Binding NewPageTemplate.LabelHeight, Mode=TwoWay, Converter={StaticResource LabelSizeConverter}}"

This is defined as:

    public class LabelSizeConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return value;
        }
 
        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            double size = (double)value;
 
            return Math.Round(size, 1);
        }
    }