Fwiw So far I've found that of all the aspects of GCSE maths, mental arithmetic is the most useful operationally.
For the ATPL theory it's useful to be able to reality check your answer before using the whizz-wheel or calculator.
Now I'm doing the CPL it's useful to be able to do on the fly revisions of eta on the basis of an estimated ground speed on a divert, plus estimating fuel demands and time remaining.
Can't speak for true commercial flying but maybe someone else will.