For us it's a set calculation based on your (gross) yearly salary. The longer you've been with the company, thus the more you earn, the more you get paid. All laid down in our Schedueling agreement. All in addition to the normal basic salary and sector pay for that day.
If you're then disrupted during that day you get a disruption payment on top of it. (half a day off payment) All bounded by set rules and regulations. I'm only with the company for 2 years as F/O and am looking at 240+ ish for a WDO. Captains around 150% of this.