On a simple form (mine are never simple) describing a picture frame and calculating and displaying it's cost, is the convention too:
Multiply the length of material required x the cost per meter in a form's on change event and have the value entered into a bound total control or make the forms underlying query do it.
Private Sub Form_AfterUpdate()
Total = ((2* Height) + (2 * Width)) * Cost
Or would you put the same thing in the underlying query?
Total:((2* [Height]) + (2 * [Width])) * [Cost]