Forms :: Calculations In A Form

performing calculations in a form in an Access 2007 database.The form is used to generate invoices and sales receipts. At the bottom of the form there are 3 text boxes: "Sub-total" (Text128), "VAT" (Text130) and "Total" (Text132). Each text box is bound to the Control Sources "Sub-Total", "VAT" and "Total" respectively in a table called "Orders".

I would like the "VAT" text box to automatically calculate the VAT on an order and enter it into the VAT field in the table "Orders". To do this I have tried using the formula:

Code:
=[Text128]*0.2

I would also like the "Total" text box to automatically calculate the total cost of the order and enter it into the Total field in the table "Orders". To do this I have tried using the formula:

Code:
=[Text128]+[Text130]

I know that these calculations are correct as they work in an unbound text box however I cannot get this to work with bound text boxes.Is it actually possible to do this or do I need to use another method to perform these calculations?