Conditional Calculation if Box is Checked

I'm using Adobe Acrobat Pro 2017.

I want the form to calculate an amount (bonus) if a specific box is checked. If the box isn't checked, then no calculation is required.

So far I have: event.value = this.getField("Salary").value * 25 / 100.0 which calculates the bonus (field called BonusTotal) as soon as a salary is entered, however, I don't want to see a bonus if the checkbox (called Bonus) is unchecked. What am I missing? Thank you!