Strangely, I haven't found a lot on that one on the internet (mostly videos clearly geared towards school children).
My issue: I am good at case math and good with numbers in general. Where I do have an issue - depending on the numbers - is to estimate a percent increase / decrease on the fly. This can be super useful to interpret a number quickly after you come up with it (and where it would sound ridicolous to take another 20 seconds to calculate it fully through in your head).
Ofc I know the typical tricks (i.e. if multiply the denominator close to 10 or 100 etc.) but sometimes I get really weird numbers, such as 37/587 where it is not really obvious at first. The only thing I could deduce here is tht it is between 5% and 10% (since 5% of 58 is 29 and 10% 58)
When I interview candidates I realize that flaw as well, i.e. most gravitate towards “it is a 50% percent decrease” even though when its much closer to 40%.
How do you handle that in general?
