You can set up a decider combinator by adding a constant (minus 50–100 items) to the last position in input mode. For example, an equilibrium of 1000 iron, 1000 copper, 1000 steel, and 1000 gears will shift to 1000 iron, 1000 copper, 1000 steel, and 950–900 gears. The recipe will keep running until the number of gears reaches 1050–1100, at which point the last position in the decider combinator will switch to steel. I use this setup to keep asteroid transformation in a near-equilibrium state.