notice in lieu computation

I have tendered my resignation on 19 June 2012. Instead of giving one month’s notice (ending 18 July 2012), I am only giving one week’s notice. How do I compute the amount owing to the firm for the three remaining weeks?

The notice in lieu to be paid is computed using the following formula:
= [(12 x monthly gross salary) / (52 x number of working days required to work in a week)] x (working days for the remaining notice period not serving)