The percentage criteria by CBSE is “the marks of that additional subject are considered in which one scores higher”.
The CBSE board has a best of five rule in which your main percentage is decided by one language subject I.e. English and other 4 subjects in which you get high scores. And the remaining subjects becomes additional whose marks won’t be added to your main percentage.
The subject you would score lowest will automatically excluded . So, if you score higher marks in Physical Education than any of your subject other than English then it would automatically replace the lowest scoring subject .
So, your percentage should calculated by adding marks of English(Mandatory) , Physics , Chemistry , Biology and Physical Education . As you said you socred lowest in maths it would be replaced by Physical Education .