I don't think there's a standard single way to do it, and the most common approach may vary by country as well.
However, I am interested to hear about how you calculate a 30% difference...
Starting 1/11, using the calendar days method gives you 21/31 or 67.7% of a month's salary, and using the working days method gives you 15/21 or 71.4% of a month's salary, which is a boost of about 5% not 30%.
Moreover, knowing this is the way they do the calculation can allow you to 'catch up' on the end of employment side of things. For example, if you leave on 5/3/2021, you would get 3/31 of a month's salary (9.7%) instead of 1/21 (4.8%).