How Exactly Do You Need to Place a Musical Hit Point?

From my earliest attempts at writing music to film back in the days when work prints were delivered as VHS (God, I feel old) I was wondering how exact do I need to be with hit points.

Back then it was technically extremely tricky to sync any video to music as normal computers were not powerful enough to run both at the same time plus video files were still way too big to handle comfortably let alone send over the internet. So I needed to do all calculations by hand referencing the visual time code on the VHS (stepwise searching for the frame that had the hit point) and then calculate that in the score at the tempo I was using as Timecode functions were only very rudimentarily implemented in software like Sibelius yet. The easy things you can do now by just slowing down a few bars by 1bpm to hit the hit point better were time consuming processes back then.

The rule of thumb is that the general audience will notice a discrepancy of +/- 2 frames between music and video already. But is that actually as exactly as you need to be?

In general, only hit points that have a sharp visual action or sound need to be as exact. For instance imagine a gun shot, a punch or a cut. 

However most actions take a certain time, for instance imagine someone stumbling. This is nothing that happens in just one frame but it might take several frames or even seconds. In general most movements take a certain time. In these cases the +/- 2 frame rule doesn't need to be followed as strictly.

In some cases the event that you're trying to hit with music spans over such a long time that you need to consider where in this event you want to place the musical hit. Imagine a big explosion with a fireball that expands and collapses again. In a movie this could easily last several seconds, so the question would be whether to place a sharp hit point on the beginning of the explosion or rather keep a sustained climax throughout the entire explosion or even stop the music before the explosion.

Many hit points, particularly the ones that take some cognitive processing by the audience feel better when they are slightly late, as the moment of the audience realizing and the music reacting align. For instance imagine a dialogue sequence where one character reveals a big plot twist to the other character. Placing any musical reaction right on the revelation might feel overpaced and reduce the impact rather than support it.

In any case, use your feeling rather than mathematically correct numbers to determine whether a hit point sits right or not. The advancement of technology allows to quickly check slightly different placements of hit points and compare them with each other so use this to find the placement that feels best from a storytelling and musical approach.