Usually, it should be calculated as below Total land area divided by Total super built up area (sum total of area of all the flats) multiplied by particular flat's super built up area.
For example No of flats 128 Built up Area of Each flat: 64 flats with 1300 sqft each and 64 flats with 1600 sqft each Total area of all flats: (64 x 1300) + (64 x 1600) = 185600 sq ft Total Land area 64000 sqft Undivided land share for 1300 sqft flat will be 64000/185600*1300 = 448 sqft.
The apartment you are booking, the sqft area of your apartment clubbed with the car parking if you are getting any will be within our ownership. Only the amenities will be shared among all the apartment owners.