Plot Ratio Calculation

The principal is that there is only 1 building height no matter how many buildings are there on the site. It should be that of the tallest building.
So in your case, building height is that for the domestic building, i.e. over 61m.
From 1st schedule of Building (Planning) Regulations, permitted P.R. for domestic and non-domestic buildings over 61m and on a Class B site are 9.0 and 15.0 respectively.
But it doesn't mean that both buildings can achieve that permitted P.R. Calculations should follow that specified under B(P)R 21(2).
If, say, the P.R. of the commercial centre as designed is X, then following B(P)R 21(2), the max. P.R. for the domestic building shall be calculated as follows:
Max. domestic P.R. = 9.0 x [(15.0 - X) / 15.0]
i.e. If the commercial centre used up the 15.0 P.R., then the development potential is fully utilized and you cannot have the domestic building, or vice versa.

The existing building is class A site. height is 95m and plot ratio is 6. Site area is 42000 SF. If I need to redevelopment of existing building includes 3 carpark floor in basement, around 40 office floor.
How to calculate plot ratio for new building ?

Site Coverage = (The area of the building / The total area of the site) x 100%

Plot Ratio = Total area of number of floor for the building / The total area of the site

For example: the building 15 floor and 100 meter square. The site area is 500 meter square

Then the site coverage = (100 / 500) x 100% = 20%

Plot Ratio = (15 x 100) / 500 = 3