I disagree with balancing AFTER the furnaces. Your throughput will still essentially be one lane of belt. The only differences is that the belt will have a larger buffer/storage. If you want to increase throughput, you need to directly output to both sides of the belt. I also suggest, if you want 10...

Honestly, if I want loads to be even, I got into the habit of ensuring that at the source - aka, if I want an even belt, I'll put a miner on either side, or smelters on either side. This is because if you have one full track, you gain very little benefit from evening the belt, as now you still only ...