This blog is very skeptical of the ability to measure THC accurately enough to tax it. Here is an argument that testing is accurate enough: “Skeptics say it is too difficult to measure THC, but that’s not very convincing when High Times magazine offers the same kind of product specs you’d find in an issue of Stereophile or Car and Driver Magazine.”
But even claims of horsepower have been found inaccurate. And nobody taxes horsepower. The sampling of bud leaves the door open to manipulation and abuse – by taxpayers or government.
Accuracy for taxation is not the same as accuracy for journalism — or advertising. And if the incentive shifts to make low THC claims profitable, I would not count on taking those low claims to the bank, either. That’s where I look to a stated THC tax as an option, as suggested by Beau Kilmer and David Ball, described here at page 541 note 17. Remember what happened to statements of tar and nicotine content on cigarette packs: they are not there any more.