@[deleted] - You might want to have a look at (this thread).
I view automatically generated "service time" as a bit of an inexact science, as it requires that certain assumptions about staff usage be made; e.g., that the staff member is working exclusively on a given, assigned ticket from the time it is assigned to the time it is closed (or possibly a little more "smartly", that the staff member's time is always equally divided between their open, assigned tickets).
I'll admit that manual entry won't be for everyone, though, as the possibility for misrepresentation of time spent - unintentionally or otherwise... - is certainly present.