The amount of cannabis tinctures you can add to your THC brownie depends on your personal preference and tolerance. Generally speaking, you should start with a low dose and increase it gradually until you find your optimal level. A common rule of thumb is to use 10 mg of THC per serving of brownie. You can use a dropper or a syringe to measure the tincture and mix it well with the butter or oil before adding it to the batter.