Overtime In Semi Monthly Timesheet

I’m trying to take an existing employee time sheet in Excel (Office XP) that has no formulae whatsoever, and add the appropriate formulae so that all an employee needs to do is enter the daily start and end times and the time sheet will calculate daily, weekly, and overtime hours worked. Among others, some of the problems I’m having are:

I need to keep the original format (though I've added a few columns).

Overtime in the State of Texas does not apply until after 40hrs have been worked. Then any daily hours over 8 can be applied retroactively. So I need a timesheet that shows overtime as regular hours worked until 40 hours have been reached, then separates the daily overtime from the regular column and places it in a daily overtime column. Shouldn't be too hard to find...Right?... Actually, that’s been quite easyexcept for weekends. Saturdays and Sundays are usually overtime but not always.

The real problem is the beginning day of the pay period, if a pay period begins on any day other than Monday (Wednesday, for example,) then weeks one and sometimes three can never equal 40 hours each unless the assumption is that the days worked in the same week but prior or subsequent period are worked at 8 hours each. The formulae must make this assumption. How do I write a formula that assumes an empty cell actually has a value? :o

I know that it’s difficult (if not impossible) to offer any suggestions without seeing the time sheet itself, so, If it would be helpful, and anyone has any suggestions. I’ve uploaded the week one of the timesheet as it stands now.

If you'd like to see the entire worksheet I've uploaded it to ....