Your contributions are calculated based on the payroll method your employer uses. It'll be one of the following three:

1. Qualifying Earnings: This method takes into account all your earnings, including bonuses and commissions. But only the earnings between a lower and an upper threshold are considered. So, if you earn more than the upper threshold, that extra bit won't go into your pension.
2. Basic Pay: With this method, your pension contributions are based solely on your basic pay. There are different sets of basic pay, but only your basic pay is considered for your pension.
3. Total Earnings: This one's pretty straightforward - all your earnings, from your salary to bonuses and commissions, are considered.

To find out which one applies to you, ask your employer. Someone in finance or HR would be best.