How to leave textbox empty to execute task

See more:

ASP.NET

protected void Button1_Click(object sender, EventArgs e)
{
SqlCommand cmd = new SqlCommand();
cmd.CommandText = "insert tbstudent values(@q,@w,@ww)";
cmd.Connection = con;
cmd.Parameters.Add("@board_m", SqlDbType.VarChar, 50).Value = TextBox9.Text;
cmd.Parameters.Add("@q", SqlDbType.VarChar, 50).Value = TextBox10.Text;
cmd.Parameters.Add("@w", SqlDbType.Float).Value = TextBox11.Text;
cmd.Parameters.Add("@ww", SqlDbType.Float).Value = TextBox12.Text;
float m1 = Convert.ToInt32(TextBox11.Text); float m2 = Convert.ToInt32(TextBox12.Text);
float m3 = (m1 / m2) * 100;
cmd.Parameters.Add("@per_m", SqlDbType.Float).Value = m3; Label1.Visible = true; Label1.Text = Convert.ToString(m3);
cmd.ExecuteNonQuery();
cmd.Dispose();
}

Suppose i have 4 fields:
Name: textbox
Class: textbox
Marks obtained: textbox
Maximum marks: textbox
%age: label1
If the result of student id not declared then it should leave blank to the textboxes
else
Its %age is displayed in label1

please help me.