Hi Folks - as usual - pulling my hair out!!

I have a report based on a query, and I am trying to change the control
source of a specific field on the report to one or other of the query result
fields, dependant on other criteria. Been 'round the houses' numerous times
now with this, but keep getting the error that the 'Field' cant be found.

Using Select Case here's where I am at (its in OnOpen):
'***************************
Dim SelectedScenario As String

SelectedScenario = DLookup("[ss_selected_scenario]", "system_settings",
"[ss_selected_scenario] = True")

Select Case SelectedScenario

Case 1

   Me!ex_calculation_factor = [bs_extent_used] ' Here's the error

End Select
'***************************
Tried numerous syntax to refer to the field and get either and Object error,
or the mentioned 'cant find the field' error.

Tried this in OnFormat too but the error said I couldn't change the record
source once the report was open!

Suggestions appreciated.