Button Press Enters A (calculated) Figure On Form?

I'm sure this is really simple... apologies, but a search hasn't thrown up any clues.

I have a table with Invoice Value, and Commission Value in it. On the associated form I enter the Invoice Value, and also the Commisison value [which 99% of the time is set at 20%.

I am trying to make a button which, when pressed, enters the calculated commission in the form field (and underlying table) - I realise this could be in the 'On Click' property on the form, but in VBA I am after something like.

Private Sub btnCommCalc_Click()
tblSales.Commission = tblSales.Invoice * 0.2
End Sub

Could someone help me with the syntax? (I can then move on to using a variable, rather than hard-wiring the 20% ;-)

Many thanks