How do you determine obstruction?

[Dakota]

For there to be obstruction, the runner must be impeded. This is umpire judgment, but the umpire must have something on which to base his judgment. The physical location of a defender is not, in and of itself, impeding a runner.

From the February ASA Rule Clarifications:

Rule 8, Section 5B: Obstruction

There seems to be some question on the ASA obstruction rule as discussed in Rule Supplement 36. It states that “If a defensive player is blocking the base or base path without the ball, they are impeding the progress of the runner and this is obstruction.” This has been interpreted by some to say that regardless of the location of the runner or runners blocking a base is obstruction. Therefore, regardless of where the runner is, for example 20 feet from the base, in ASA this is obstruction. This is a misunderstanding of the Rule Supplement. The sentence is being taken out of context and should be applied with the rest of Rule Supplement 36.

However, it is also important to remember that a Rule Supplement is not a rule but written to support the rule. Our rule clearly states that it is obstruction if there is impeding of the runner. To have obstruction while blocking a base there must be two elements involved: 1) A defensive player blocking the base or base path without possession of the ball and 2) a defensive player impeding or hindering the runner’s advancing or returning to a base by the action of blocking the base.

Ruling: No. R1 was not impeded or hindered on the way to the plate. (Rule 1, Definition; Rule 8, Section 5B)

Merely potentially being in the way is not sufficient. Personally, I don't have a big problem with what you stated in your original post, where once the defender remains blocking as the runner and defender reach close proximity, you judge that as impeding the runner. But, I look for some physical indicator that the runner did something in order to avoid, etc., the defender, rather than proceed directly to the base