Here is an interesting quote from a concurrence by Judge Danny Boggs, U. S. Court of Appeals for Sixth Circuit:

"While courts have resisted mightily putting a number on probable cause, see Maryland v. Pringle, 540 U.S. 366, 371 (2003), at bottom a review of cases indicates that there must be some, albeit inchoate, feeling as to what kind of probability constitutes probable cause. My reading is that it does not require a belief that there is more than a 50% probability of evidence being found in a particular location. See, e.g., United States v. Gourde, 382 F.3d 1003, 1015 (9th Cir. 2004) (Gould, J., concurring) (collecting cases). If that were the case, one could never get a search warrant to search all three cars of a person for whom there was overwhelming evidence of general drug dealing, and specific evidence of a drug transaction the proceeds of which were now certainly in one of three cars in his garage, and certainly not in any of the others. However, to be more than a hunch or a supposition, in my own mind, requires a legitimate belief that there is more than a 5 or 10 percent chance that a crime is being committed or that evidence is in a particular location."