The regular rate
The regular rate includes most bonuses and shift differentials, not only the base hourly rate. Overtime is paid at 1.5 times that blended rate.
Workweeks, not pay periods
Overtime is counted per seven-day workweek. Two 45-hour weeks in a biweekly period mean 10 overtime hours, even though the period total is only 90.
State rules
California pays daily overtime over 8 hours and double time over 12. A few other states have their own rules.