What is the test for determining the net weight of a piece of cocaine seized by a police officer at a traffic stop?

Respondent contends Officer Spear's testimony was unreliable as it was based on his determination the chunk weighed .1 gram; whereas, the Department of Justice determined the net weight was actually .068 grams. As previously set forth, it is not the quantity of narcotics but the usability of the narcotics that will determine whether a usable quantity will be found to exist. (People v. Rubacalba, supra, 6 Cal.4th 62, 66 ["the amount of cocaine needed to produce a narcotic effect and the purity of the substance need not be proven in order to establish a usable quantity"].)