C# code:

    public sealed class DoublesToTooltipConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type t, object parm, CultureInfo lang)
        {
            if (values.Length != 2 || values.Any(x => x == DependencyProperty.UnsetValue))
            {
                return null;
            }
            if (!(values[0] is double v1) || !(values[1] is double v2))
            {
                return null;
            }
            string p1 = Tools.Normalize(v1);
            string p2 = Tools.Normalize(v2);
            if (v2 == double.MaxValue)
            {
                return $"{p1} used";
            }
            return $"{p1} used out of {p2}";
        }

        public object[] ConvertBack(object value, Type[] t, object parm, CultureInfo lang)
        {
            throw new NotImplementedException();
        }
    }