Calculate overpay hours and salary extra rates for specific time ranges

Hi

I have gotten quite  headache tryng to figure this out.

Lets say I have a 2 fields:
- Work Start
- Work End

If the employee works between 22:00 and 06:00 they are entitled to an extra 10% in salary for those hours.

The employee works from 21:00 to 06:00
How do I calculate the amount of hours that the extra 10% should be paid? 

I would separate normal rate and overtime rate as fields. Count normal rate and then count over time rate. Then total day pay is an equation that calculates rate.

All the best,

Ramsey
Ramsey@garmar.co.uk