I believe Hornady makes a sort of universal 30 cal neck sizing die.  The die is available from most suppliers(if in stock).  If you have a 308 Win die you can use that, Install die with about a 1/4 inch clearance from the shell holder and run a case into it.  You will see an obvious line where it stopped sizing the neck.  Adjust until sizing stops at the juncture of neck and shoulder.  A FL die works, the 308 case is fatter and will not contact the sides of the case.  Backing out your FL die until it just sizes the neck will work also.  The advice about annealing is spot on, it is easy to do and will extend case life, an issue since as you have found brass for a Krag is not easy to come by.