Use the combincation of bradaccess and mm-uk solutions - have the textbox with the calculation in enabled=no and loicked=yes and a textbox with the height and width set to 0 (but enabled=true and locked=true.
Alternatively have a command button on the form with the focus set to that.
Or if the calculation is for display only, use a messagebox with the calculated value on - numerous of ways of doing this - refer to a hidden control on the main form with the calculation in, perform the calculation in code etc....