Dear Himanshu,
I would like you to read the explanation given to sec.4(2) of the Payment of Gratuity Act,1972. The explanation clearly states that in case of a monthly rated employee, the 15 days' wages shall be calculated by dividing the monthly rate of wages last drawn by him 26 and multiplying the quotient by 15.
Therefore, as far as the formula for calculation of gratuity under the Act in respect of any monthly rated employee is concerned, the no. of his weekly working days is not relevant.
If at all it has got any relevance under the Act, it is only for the purpose of determination continuous service u/s 2-A of the Act. In case of an employee who is employed in an establishment which works for less than 6 days in a week is not in continuous service as per sec.2-A (1), he should be deemed to be in continuous service if he has actually worked for 190 days in a year or 95 days in a period of 6 months as per cl (a) and (b) of sec.2-A (2).