Take your total monthly “wage” (including salary and regular allowances as defined in Art. 2, subject to the exclusions listed in Art. 10 of the Enforcement Rules), divide it by 240 (30 days including weekends and holidays, 8 hours per day), and you have your theoretical hourly rate on which to base the Art. 24 overtime rates.
This is different from the “average wage” formula, in which the length of a month is supposed to be more than 30 days. It also creates a discrepancy between the basic (aka minimum) wage for full-time and part-time workers. It’s $20,008/m or $120/h, but with the monthly version the theoretical hourly rate is $83.37/h.
Also check the fine print of your contract and work rules. If either of them contains a formula more favorable to you, it prevails.