Whether or not you use more than the minimum amount of developer to cover your reel depends on the developer you use and its dilution. The rule I use when experimenting with a new developer or with extended dilutions - use at least 100ml of the stock solution (ex. D76) per 80 sq in of film or 100ml of the lowest dilution for which the manufacturer provides times (ex. Rodinal 1:25). Using these guidelines I don't believe I have ever underdeveloped a roll for lack of active developing agent.