Compact smelting setup able to fully saturate one blue belt using less than one blue belt as input.
One belt of iron or copper ore is first fed into a lane balancer ensuring input lanes are consumed evenly. This can be removed to save some space.
The first twelve smelters almost saturates a blue belt. The 13th smelter has buffer belts feeding both lanes guaranteeing a 100% saturated belt.
Now 42x13 thanks to DaveMcW's tweak