TextBox in UserForm to show cell value instantly

I have two TextBoxes (txtSpan & txtTrib) in a UserForm (frmSizes), when values are entered in these TextBoxes they show (as they are typed) in cells of the spreadsheet. The resulting calculation is in cell B11.

How do I get the value in cell B11 to show in a third TextBox (txtAns) as soon as it is calculated without closing the UserForm or running another code?

Thanks Sandy