public string ParsePercent(string expression)
{
    var list = SplitAndKeep(expression, expression.Where(x => x != '%' && !char.IsDigit(x)).ToArray()).ToList();

    if (list.Count > 0 && list[0].Contains('%'))
    {
        list[0] = list[0].Replace("%", "");
    }

    for (int i = 1; i < list.Count; i++)
    {
        if (!char.IsDigit(list[i][0]))
            continue;

        if (list[i].Contains('%'))
        {
            list[i] = (int.Parse(list[i - 2]) * int.Parse(list[i].Replace("%", "")) / 100).ToString();
        }
    }
    return string.Join("",list);
}

public static IEnumerable<string> SplitAndKeep(string s, char[] delims)
{
    int start = 0, index;

    while ((index = s.IndexOfAny(delims, start)) != -1)
    {
        if (index - start > 0)
            yield return s.Substring(start, index - start);
        yield return s.Substring(index, 1);
        start = index + 1;
    }

    if (start < s.Length)
    {
        yield return s.Substring(start);
    }
}