Custom Calculation Script for Adobe Acrobat

I have a timesheet with four columns: 1) Time-In, 2) Time-Out, 3) Lunch Hours and 4) Work Hours. A user enters values for Time-In and Time-Out (both in hh:MM tt format). The difference in the number of hours is computed (initially) under Work Hours. The user can then (manually) enter a value (in hours) under Lunch Hours. This number is then automatically subtracted from the (initial) Work Hours. A new value for Work Hours is then automatically computer. For example, user enters "08:00 am" for Time-In and "08:00 pm" for Time-Out. The value for Work Hours is automatically computed (initially) as "12:00" hours. User then enters "2.00" for Lunch Hours. A new value of "10.00" is then computed for Work Hours. I have attached a sample pdf to illustrate the timesheet.