Unsure about home office calculation

My home office is 3m by 4m. I am not sure how to calculate the rates and electricity for the office?

Kindly refer to the example below:

For example:
Rates (R2,400) Electricity (R1,500) = R3,900
House is 60 square metres
Office is 3m x 4m = 12 square metres
Portion = R3,900 x 12 / 60 = R780.