Do we need to recalculate overtime when offering an annual bonus?

Q: We have mostly hourly, part-time, shift-based employees. We instituted a once per year small discretionary loyalty bonus. If the amount is fixed, for example $100 per year of employment, some people tell us we need to recalculate the overtime rates for all 26 prior periods (which will amount to pennies, but cost us hours to do). It is impractical in a very small (12-18 employees). We are looking at calculating the bonus at 0.2% of total past year compensation. For example, for one employee, it will be $86.13. Can we round it with a discretionary bonus of $13.87 to get a round number bonus of $100.00? Are there other options to give a reward without the whole pain of overtime recalculation? - Alan, Tx.