Distribution of expense between different society.

Hi 

Builder has constructed four building and created four  seperate society of each building. Now A,B building have 88 flats C and D building have 43 and 60 flats, common expenses like electricity,manager payment,water bill, etc are share according to percentage .

Eg if bill is 10000/no of flats multiple by building flats.

My question is it legal to division and how should it be legally divided. 

Thanks .