Stated in statistics terms, the lower the confidence interval around how much a stock is worth the less you should own (a smaller position size) and the less willing to add to a position you should be. I sorta learned this principle when Einhorn commented that he was avoiding HLF (a 0% position size) because "the range of outcomes is too wide" to get involved.
I'm no statistics expert so I'm not sure my use of the term 'confidence interval' can be applied to this situation. I'm trying to expand on the Einhorn idea because I don't think he fully captured it. In every stock the range of outcomes is wide, every stock could go to $0 or to some really high number. The key factor is the range of RESONABLE outcomes. Whether there is a credible case for the stock to reach those levels. So, to restate the rule: The wider the range of resonable (likely) outcomes, the less the investor should own of it and the less willing to add a position the investor should be
How to determine the range of likely outcomes? You got to read the long case, read the short case, know a lot about business, look at the capital structure, try to infer things from insider behavior, the industry etc etc. Its all one big guesstimate
But of course, one can't take the rule too far. If there is a wide range of likely outcomes but the stock is trading at close to one of the extremes, it might pay to go the other way. VRX is either a $0 or worth a lot more than the current price. It has gotten beaten down so badly by the short thesis that I just think its worth going the other way because the wide range of outcomes is actually good when the stock is at one of the extremes. Its like owning a mispriced option
This might be why Einhorn covered his HLF short and stayed on the sidelines after the stock collapsed. The stock went to an extreme (to the downside after the Ackman presentation), he had the chance to take a profit at one of the extremes. Owning a short at that level no longer made sense so he took it