The forming apparatus of the present invention preferably includes an orifice on top of the trough. The uniquely shaped orifice substitutes for the weirs as the controlling entity for glass thickness. The orifice is designed such that as it deforms, it maintains a linear flow characteristic with respect to its length. As the orifice is made larger by the applied stress, the percentage width increase is the same over its length and consequently the percentage flow increase is the same over the length of the orifice. In another embodiment, the present invention provides an adjustment to change the flow characteristics of the trough to compensate for the degradation of the forming trough during an extended production run. A flow control plug can be inserted into the trough, such that flow dynamics can be altered during hot operation by insertion, removal or position adjustment of the flow control plug.