Ollie,
  I'm assuming TotalCost is an entered value, not calculated?
  Given that premise, it's really not necessary to save the TotalCostPct.
As a general rule, don't save a calculated value that can always be
re-calculated from existing values... in any subsequent form, query, or
report.

  Ex. Given Price * Qty = Line Total
  Since you capture Price and Qty, you wouldn't want to save LineTotal.  It
can always be recalculated from Price and Qty (on the fly) in any subsequent
query, form, or report.

   If you MUST save the TotalCostPct, create a field in your table called
TotalCostPct, add that bound field to your form, and use the AfterUpdate
event of TotalCost to update the TotalCostPct field.
   Private Sub TotalCost_AfterUpdate()
       TotalCostPct =([Total_Cost]/100)*40
   End Sub
   Whenever TotalCost changes TotalCostPct will be updated.

In general, it's recommended that you do not store calculated values. As
fellow MVP John Vinson likes to say, "Storing calculated data generally
accomplishes only three things: it wastes disk space, it wastes time (a disk
fetch is much slower than almost any reasonable calculation), and it risks
data validity, since once it's stored in a table either the Total or one of
the fields that goes into the total may be changed, making the value WRONG."

You can always put the calculation into a query, and use the query wherever
you would otherwise have used the table.

If you're insistent, you have two choices:

1. Bind the text box to the field in the recordset and set its value through
VBA
2. Use the calculation as the control source, and set the field in the
recordset through VBA (usually done in the form's BeforeUpdate event.