A seed metering gate assembly (12) is provided for use with an upright seed bin (10) having a shiftable primary gate (24), wherein the gate assembly (12) provides an additional seed flow control function allowing more accurate dispensing of seed from the bin (10). The gate assembly (12) includes a seed hopper (13) equipped with a fore and aft-shiftable metering gate (42) movable between a closed position and an open position presenting a seed output passageway (40). Apparatus (62) is operably coupled with the gate member (42) in order to determine the travel of metering gate member between the fully closed position and a plurality of open positions presenting respective, differently sized passageways (40). The apparatus (62) preferably includes a rod (64) having a series of through openings (70) and coupled with the gate member (42), with a shiftable, motion-limiting collar (72) mounted on the rod (64) and securable at any of the openings (70) by a clip (76). The opening movement of the rod (64) is limited by engagement between the collar (72) and a bracket (68) affixed to hopper (13).