Bonus for persons working less than full time hours is calculated according to the setting set in the Employment Type. If this is set to pro-rata bonus the system will calculate it according to the number of hours worked.
The system will check how many hours the employee worked from the previous bonus (6 months earlier), and calculate the pro rata depending on the hours.
The exact info how the bonus is calculated is by clicking the (i) button right near the Bonus amount. Below is an example of an employee who worked 30 Hours per Week: