The amount of CBD in a joint depends on the concentration of CBD in the strain and how much flower is used to roll the joint. If a strain is 15% CBD and the joint is 1/4 gram, then 150 x .25 = 37.5mg CBDA x 0.8 = about 30mg CBD. Then you may absorb anywhere from 2-56% of that amount. If more flower is used, or the concentration is higher or lower, the amount will be different. I would not depend on accurate dosing by rolling joints. Tinctures and tablets are more accurate when it comes to dosing.