Some sources will give you a number. How accurate that is, is anybody's guess. I figure between 15 and 20 mg. Which is based on nothing but my imagination. The best way is by gauging the effects. My wife has a very low tolerance, so 5 ml and she's done. Too much and its all anxiety. So I don't try to make things strong. It just comes out that way.
You lose something like 25% of THC when smoking. I'm guessing there is quite a loss when decarbo'ing as my temps are a bit sloppy, even using all digital. Am I getting 90-100%? No way to tell. And a loss in digestion. So what you start with is not as significant as the effects you get out.
There was a good article posted on GWE a month or two ago about THC testing in Oregon. Bottom line is tests are inconsistent and testing adds costs to legal growers, allows for mislabeling of THC content, which can raise prices to consumers. Blind testing mentioned in the article really pointed out how variable the current tests or testing labs are.
Try one gram of flower to one ml of liquid and see how that works for you.