What is the test for estimating the quantity of drugs seized in a criminal case?

"The quantity of drugs attributable to a defendant is a question of fact," and "if the evidence-direct or circumstantial-supports a district court's preponderance determination as to drug quantity, we must sustain that finding." Jones, 531 F.3d at 175. In a drug case, the Guidelines provide that where the "amount [of drugs] seized does not reflect the scale of the offense, the court shall approximate the quantity of the controlled substance." U.S.S.G. 2D1.1, cmt. 12. The court's estimate of the drug quantity must be based on "specific evidence," which has been defined as "evidence that points specifically to a drug quantity for which the defendant is responsible." United States v. Shonubi, 103 F.3d 1085, 1089-90 (2d Cir. 1997). "In making such an estimate, the court has broad discretion to consider all relevant information." United States v. Blount, 291 F.3d 201, 215-16 (2d Cir. 2002).