I did take a quick look at your PDF file.
I assume you don't want RegHours to default to 8 hours, it should be the result of the calculation if that result is 8 hours or below, and it should be 8 hours if it is above 8 hours. The overtime field should in that case be the difference between the calculated time and 8 hours.
As far as the breaktime goes, you can just subtract it from the calculated number of hours.
The reason you are ending up with negative numbers is that you are using minute values - so if somebody works over midnight, you are subtracting a large number from a small number. Because your form does not allow somebody to work across two midnights, you can just add 24 to the result if you end up with a negative value.
I made a few more changes to your form, so that you can use the same calculation script for every result field (it determines which row it is in automatically). Take a look at http://www.khkonsulting.com/files/TimesheetFillable.pdf for more details.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com