Some online video players give you the option to add or subtract delay e.g. by ±0.5s to subtitles to manually sync them to the video currently playing. However I become functionally dyslexic when it comes to actually figuring out how much delay has to be added. I end up randomly adding or subtracting delay and make it work through trial and error.
Is there some rule of thumb I could follow instead to systematically sync subtitles?
I get that but I wouldn’t know any such way. There are dedicated subtitle tools that can sync audio and text but those need the files. Failing that you can only do it by ear.
If the subtitle file was downloaded from opensubtitles automatically, there’s also the very real possibility that it’s the wrong file. It might be the correct movie but the subtitles might be synced to a different framerate and there’d be no way for you to know.