Not a stupid question at all.
It's 14% THC by mass. So if you take 1 gram of bud, you get 140 mg THC.
We take a part of the sample provided for the test, about 1/4 of the nugget or so (about 200 mg) and get the weight % of THC of that. When we have done re-tests of another 200 mg of the same...
We're a lab in MA. We post a dosage calculator with each tested sample here: mcrlabs.com/tested. As you can see, concentrates have ~60-70% THC, while bud ranges wildly between ~5% to ~20%, even for the same strain names. So the best thing to do is to follow what colonuggs' suggestions above.