Personally, I find that there's a simple question:
Does this move the company, project, or team forward in a measurable way?
If the answer is "No" then you need to take a long hard look at it and explicitly decide that it's either okay and then stop doing it (ideal) or limit it. And that varies from person to person. Spending time with my toddler is okay in my book. HackerNews, less so.
The next level question is:
Is this the most effective task to this move the company, project, or team forward?
And that is way harder to answer.
I think a better question is, "Is this task consistent with our core values, value proposition, and beliefs."
Limiting yourself to only measurable activities will cut you off from serendipity, which alone can make or break a startup.
But if the problem is lack of focus, there's a balance in there. And it's going to be different for everyone. I think that's a useful bit to remember.
Hey, look at that! It's by Jessica Livingston!
This said, it's a survey of largely succesful organisations - and focuses in the main on what people did right. This talk was about anti-patterns, that in many cases often led to unsuccessful startups (or at least put unnecessary bumps in the road).
I can completely empathize that it's hard to talk safely about other people's private failures, especially as an investor.