Consider how your tick box is going to work. I assume it isn't bound to your record source (linked to your table). So when you tick it an event such as one suggested by healdem will do what you want for that record. But what do you want to happen when you move to the next (or previous) record. The tick box will still hold the same value so if it had been ticked for the previous record, it will still be ticked regardless of whether the current record should have discount or not - so it looks inconsistent to the user. This is probably what healdem is referring to when he mentions OnCurrent. On this basis I would not use a tick box like this.
Use a button to calc and set the required value (and then perhaps have another button to clear it). You could even have a simple message to auto display if the discount is non-zero
Store the tick box (yes/no field) in your table. You then don't need to store the result [Net] (and should not anyway) as this can always be calculated based on the value of your tick box and [Gross] and [Discount].