A little late where I am and probably not the best answer due to my inexperience but how about something like this?

Code:
// Class level list containing Text boxes for calculating
List<TextBox> txtList = new List<TextBox>(10);

public frmMain()
{
   InitializeComponent();

   // Add text box to list. Only have to be added in once
   txtList.Add(txtBox1);
   txtList.Add(txtBox2);
   txtList.Add(txtBox3);
}

private void btnCalculate_Click(object sender, EventArgs e))
{
   // Run a foreach loop 
   foreach (TextBox t in txtList)
   {
      t.Text = Calculate(t.Text, 0.45).ToString();
   }
}

private double Calculate(string firstVal, double secondVal)
{
   // Do some calculation
   double calcTotal = double.Parse(firstVal) * secondVal;

   // Return double
   return calcTotal;
}
Don't know the efficiency of doing something like that though as it seems like it would get confusing fast.

The only advantage to the code I can see would be the ability to use a foreach loop and change the calculate function to something else without having to write out too much code.