A custom clothier has years of experience (and sometimes even a special school of though) what and how to measure.
IRL, you get people who miss their measurement unit (cm or inches seem to be complicated). Or they misunderstand instructions and thus just enter 10 cm while they should enter 10.5 cm.
However, there are online stores for custom made shirts (with lots of measurements) and it sometimes works, depending on the dedication on the user's side.