Vector is a term I use to describe the direction of a story. The general idea of a story, the husk, the main idea, is itself a vector. A vector, in math land, has a direction and magnitude. We can imagine this with cardinal directions, an arrow pointed in some direction, and how far that arrow extends to. If our main vector is pointed due north for 10 miles, then we better have at least 10 miles of content to support that main idea.
A story can be broken up into shorter parts like character interactions, scenes, and beats. It doesn’t matter how you divide your story up, you just have to consider the granularity at which you do. These smaller parts will eventually be added up. Like in math land, when you add vectors together you also inherit direction and magnitude. Our main idea is pointing due north, if we have a lot of content that’s pointing due south then I think we have a problem. However, if something is going north-west or north-east then then you have to consider if its worthwhile to go off topic. Sometimes going off topic is desirable to create tensions and suspense. With that said, you’ll need at least 10 miles of content to reach the point where our main idea ends. I say “at least” because the shortest path between two points is a straight line. I don’t think any good story is as bare bones as a straight line, it’ll weave in and out of the main subject matter. Maybe all the smaller vectors’ lengths add up to 15 miles. That’s fine so long as it reaches the destination set out by the main vector.
There’s no absolute formula of how far you should deviate. This is just a tool to evaluate your story and its components to see whether or not you should cut things out. This analogy, of course, an be generalized for all kinds of editing purposes like whether or not you should include that game system or not.