Create a sub in the form's module (same place as the event code) and call it
whatever you want:
Private Sub DoStuffForTextBoxes
   ... put the code you want run here
End Sub

then in the before update event just say:
DoStuffForTextBoxes
and it will call your procedure.
If you need to send a value to the procedure, you can do that as well:
Check out the help on passing an argument to a function or subroutine.

You don't actually need a BeforeUpdate procedure for each
text box.  Instead you can call your function by using
    =Functionname()
in each of the text box's Before Update **property**

If the function needs to "know" which text boxes the
function is operating on, it can refer to it by using:
    Me.ActiveControl