THC percentage just tells you out of all the cannabinoids found 15%, for example, was THC. That gives you an idea what the QUALITY of the high is.. given the tHC/CBD ratio.. but it has nothing to do with potency. Lets say a gram of bud has 10 trichomes and each trichome contains 15% THC, 2% CBD, 1% CBN and so on. A a gram of bud that has 25 trichomes and is 8%, 1.5%, and 1% respectively would probably be more potent because you are taking in more with each toke. What I'm getting at is these percentages don't tell you how much Cannabinoids are there in all.. it's just a ratio