Ken had some excellent points in his answer. The calculation is just one part of a math question in the McKinsey interview.
Before that, you need to:
- Discuss the objective and numbers with the interviewer
- Layout an approach on how you want to calculate and discuss with the interviewer
After the feared calculations you need to
- Communicate your answer top-down
- Discuss implications for the client in the context of the case
In addition, throughout all of this, the interviewer will take note of your structure, communication, the creativity of approach, maturity, etc.
There is so much more to math in cases than calculating a few large numbers. Focusing on this extensively is almost always a self-fulfilling prophecy for candidates who throw in the towel as soon as the interviewer hands out some numbers...
That being said, there is no excuse for not performing math drills for hours every day if that is a key weakness for you. Build that math muscle vigorously.
All the best (from someone who also f*cked up math during their actual McK interviews and still got in...)