If you purchase your flower at a dispensary, it will state the % THC, making it easier to determine the final milligrams. If you know that number and how much bud you've used in grams, you can input your information into my online dosage calculator to help you guestimate your final dose. You can calculate the dose for the entire batch of brownies or just a single serving. But, of course, if you don't know your starting material's potency, the final strength can be anyone's guess.