Using TextChanged event for TextBox inside GridView in ASP.Net C#

Hello, I'm a little bit confusion, how to get value of TextBox of GridView in RowEditing and I've textchange event and that textbox value need to update via gridview to database. but before updating we need to calculate that value.

public void calculationA()
{
    TextBox txt_BCICU = (TextBox)grdlist.FindControl("txt_BCICU");
    TextBox txt_BCSupDlx = (TextBox)grdlist.FindControl("txt_BCSupDlx");
    txt_TotalChargeA.Text = (Convert.ToDecimal(txt_RegisterCharge.Text.Trim()) + Convert.ToDecimal(txt_BCICU.Text.Trim())).ToString();
}

protected void txt_BCICU_TextChanged(object sender, EventArgs e)
{
    calculationA();
}

protected void grdlist_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
    TextBox txt_BCICU = (TextBox)grdlist.Rows[e.RowIndex].FindControl("txt_BCICU");
    TextBox txt_BCSupDlx = (TextBox)grdlist.Rows[e.RowIndex].FindControl("txt_BCSupDlx");
}

In RowUpdating we get value normally but in function calculationA() i'm not getting value of textbox. and need to calculate that value and show edited value in same textbox also. Please help me. It's an urgent.

I'm getting null value here. TextBox txt_BCICU is null. how can I get value of TextBox.

Thank you in advance...