Percentage Calculation in C#

public void CalculateTotal()
{
    int th1, th2, practical, oral, termwork, total;
    double percentage;

    th1 = Convert.ToInt32(txtTheory1.Text);
    th2 = Convert.ToInt32(txtTheory2.Text);
    practical = Convert.ToInt32(txtPractical.Text);
    oral = Convert.ToInt32(txtOral.Text);
    termwork = Convert.ToInt32(txtTermWork.Text);

    total = th1 + th2 + practical + oral + termwork;
    txtTotal.Text = total.ToString();

    if (Convert.ToInt32(txtTheory2.Text) == 0)
    {
        percentage = total * 100 f / 300;
        txtPercent.Text = Math.Round(percentage, 2).ToString() + " %";
    }
}