Well what you did was count 2017's flow+attrition and divide it into the whole pilot group. That means you are assuming that 2017's flow+attrition will continue for the next 6 years. Nobody knows exactly what attrition will look like, but assuming that the 824/Protected pilot flow numbers that happened in 2017 will remain in place for the next 6 years doesn't make sense.

Start with the contractual flow minimums, and treat that as best-case scenario for flow. Then add your guess for outside attrition, and there's your estimate. The union has already done that and they got around 9 years for a new hire today.