Usually for tinctures, the dose is related to your body weight. Often the bottle will state how many "dropperfuls" for person weighing 100 pounds; then 101 to 150 pounds; then 151 to 200 pounds.
You may absorb the cannabis slower or faster, depending on if you have eaten OR if you have an empty stomach.
You made need to try both ways.
Also, wait at least one hour before taking an additional dose.
The motto is "start low, and go slow".
Tinctures are considered "edibles"