Hi there,
Hope you are doing well.
Due to the difference in Maximum Marks of other state boards, the concept of best four subjects percentage was developed.
Even if the maximum marks in Punjab Board is 75, you can still easily calculate your percentage as follows:-

Percentage = ( Sum of highest marks in 4 subjects/ (4* 75) ) *100

Or simply:-
Percentage = ( Sum of highest marks in 4 subjects / 3)

Hope it would help.
Thank You