Without knowing your demand for chlorine (how much iron, manganese, H2S and if there is any bacteria and what kind) and the volume of water your house uses per day and your peak demand gpm, you may have to (read probably will have to) drop more than one pellet more frequently than once per day. Usually pellets are dropped each pump run or a run may be skipped but much more frequent than once a day.
To my knowledge there is no such thing as a manual pellet dropper and I doubt one would work because it wouldn't be able to maintain a minimum chlorine residual in the water.
I would go the Sentry over other brands/types. I don't like droppers of any kind. I like an inline chlorine feeder and mixing tank.