Tony,
You could either restore the formula in the ControlSource
programmatically when your code makes changes to cells the formula
refs, OR use the Worksheet_Change event to update the appropriate
textbox (no ControlSource set). Of course, you'd need a mechanism to
test if the userform is open but that's no problem...

If Not fMyForm Is Nothing Then _
fMyForm.txtMyTextbox.Text = Target.Value

...assumes you also checked to verify that 'Target' is the cell with the
formula linked to your textbox.

From a maintenance standpoint, I'd go with restoring the formula...

sMyLinkedCellFormula = Range("MyLinkedCell").Formula
'..make changes
Range("MyLinkedCell").Formula = sMyLinkedCellFormula

...where your procedure that changes ref'd cells can load the existing
formula into the variable BEFORE making changes, then put it back
afterward.

I recommend using defined names for the cells, scoped local to the
worksheet so there's no ambiguity as to which cells your code works
with when your sheet layout gets revised, or when rows/cols are
inserted/deleted.

--
Garry

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