Hello, The question is very simple to be honest as all my referalls are passing a rigourious time math calculation pressure and training. Despite comments the best is to calculate it in your head still the problem is that the level of stress that you will find during your interview will be very high and will cause issues to concentrate. This is why you need to practice a lot just to have a confort that the calculation methodology will be automatic for you. The time of calculation is as follow :
Beginner - 25 seconds or more
First week of pracitce - around 15 seconds
2-3 weeks of practice - around 12 seconds
4+ weeks of practice - around 10 seconds
I always recommend to enter the interview with a timing below 10s. My average score when I was entering the intererview were 6-7 seconds (None of my referalls have reached this level yet).
I hope it's practical. Let me know if you would like to meet and understand my methodology of calculation and the training approach.
Sincerely,
Oscar