What is the standard of review for a finding of fact as to the quantity of drugs seized?

"'A district court's findings as to the quantity of narcotics involved in an offense are findings of fact subject to the clearly erroneous standard of review.'" United States v. McLean, 287 F.3d 127, 133 (2d Cir. 2002) (quoting United States v. Prince, 110 F.3d 921, 924 (2d Cir. 1997)). "To reject a finding of fact as clearly erroneous, we must, upon review of the entire record, be left with the definite and firm conviction that a mistake has been committed." United States v. Snow, 462 F.3d 55, 72 (2d Cir. 2006) (quotation marks omitted). Furthermore, "[w]here there is no drug seizure or the amount 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. n.12.