Probably the easiest precise way to do it is to:
- Record something that shows the problem
- Load the recording into your favorite video editor
- In the video editor, split the video and audio into separate tracks
- Line up the two tracks so that you're happy with it
- Look at what it took to do that, and translate it into milliseconds to add to the setting above
Repeat that until the recording is right.
One thing to note is that some sources have variable latency *in the physical device* (not in OBS; OBS just takes what the device gives it). I had a set of 4 HDMI -> USB capture cards a while ago, and each card had its own variable latency. So the cameras were never lined up with each other, not to mention the audio from its own USB line-in, and they were off by different amounts each time. A single 4-input internal card fixed that, and boosted the quality too.
