There isn't any method to determine the exact number of Twitter customers that read or are impacted by a tweet. To calculate the impact or impressions of our hashtag we do a quite simple math. Since what we want to measure is the variety of times that the hashtag could have been seen we are going to go person by user multiplying the variety of followers by the number of tweets despatched. Each time that a consumer sends a tweet, he/she creates as many impressions of the hashtag we want to measure as his/her number of followers. We work on a daily basis surrounded by Twitter analytics, so what did you expect?