Working Days Counter Based on State, Lock # when completed


I am trying to create a counter that counts the number of days in work of a task, the counter only counts the days in work when the Scrum state of the task changes to sprint . I want the counter to stop counting when the state changes to done and lock that number of days in work (days in work from sprint to done) and still be able to display the number so that i can do averages for reporting out. 

This is what I have so far, it counts when the scrum state is 'Sprint-Fleet' or 'Sprint-Mfg' only; however when the state changes it doesn't count and goes to blank... any tips?

=IF(OR([SCRUM State]472 = "Sprint-Fleet", [SCRUM State]472 = "Sprint-Mfg"), TODAY() - [Sprint Initiation Date]472, "")


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