You want to focus on the rules that limit your game to a few distinct possibilities. For example, you might get a rule like this one on a linear game:
X is either 3rd or 4th.
This neatly divides my options into two, and I will make a diagram for each. One represents X in 3, and the other represents X in 4.
A "block" is another rule type that lends itself to making scenarios. For example, you might see something like: X is two spaces after Y.
X _ Y
You'll notice that this block takes up a lot of space. There may only be 3 or 4 places for it to go. If that is the case, I will create a scenario for each option.
My personal policy is this: If I can find a rule that gives me 4 or fewer distinct possibilities, I will diagram them all. I generally do not want to make 5 or 6 diagrams before starting the questions. 4 or fewer is about right.
Consider a rule like this one: X is before Y.
X - Y
Imagine there are 6 spaces in this linear game. There are a lot of options that will allow X to be before Y. This is not a rule that I will create scenarios for, because I don't have time to draw a million diagrams.
There are certain grouping games for which multiple scenarios can be helpful. For example, imagine that we are placing 6 letters into two groups, X and Y. All letters must be included.
ABCDEF
X:
Y:
Rule: A and B cannot be in the same group.
Because there are only two groups, this creates two distinct possibilities, and I will diagram both.
X: A Y: B or X: B Y: A
I will then consider my other rules and fill in these two diagrams accordingly. In short, you want to look for rules that create a few distinct options.