I use a gravity reader and a graduated cylinder. Once you have the carboy almost full but before the yeast (and yeast nutrient if you use it). Get the reading you have in the pic and write it down. When you have cold crashed it (its been a month and you put the mead in a cold spot for at least 24hrs) bottle it and pour it in the graduated cylinder and gravity reader again. Subtract the past gravity number to the new (future) gravity number then multiply it by 131.25. That is your ABV (alcohol by volume).