What do you mean by "how's pro rata worked out"? Do you mean you've worked a part -month during your notice period and you want to know how your pay is calculated?
It would normally be based on the number of days you've served during that month. So if you work full time, and do two weeks in the final month, you'd get paid for 10 days pay. One day's pay is usually your annual salary divided by 260.
But there's no point me saying what usually happens as all of these can vary, either because you don't work full time or regular hours, or your employer may use a different method.