The ANSI standard is ~30% like it or not. At 500 more like 50%. That;s 1/2 stop either way, a total of one stop.
Adjusting a LF shutter can be pretty tedious. The slow speeds are controlled by the governor itself, clean it, lube it. and by the POSITION of the governor relative to the retard lever that it engages.
High speeds can be adjusted by replacing main springs or swaging/filing sections of the speed cam. suggesting this is a simple thing doesn't suggest familiarity with the work.
Adjusting a metal plate requires a calibrated hammer & punch and filing can be one stroke +/- sorta.