Checkbox to update value in calculated control?

[PmF]

I have a checkbox called Parking on a form called frm_Booking and when checked it needs to update the AmountDue field on the same form by adding £10 to the value.

How do I do this?

 

[nickjar2]

on the after update of the check box try putting:

if parking = -1 then
amountDue = amountDue + 10
end if

Nick