I'm a fan of flow by priority, and use smart splitters to overflow to the next priority. Let the belts balance themselves once production becomes saturated.
Whatever line needs to be up and running first, send full belts. Install smart splitters on those lines and designate an overflow direction. Merge those overflow belts into other belts, rinse and repeat for following belts. It will auto balance once the intakes of the machines fill up, and your material production can keep contributing to active production. Example I found online
Another option is to use the double size storage containers to act as both buffers and automatic mergers/splitters.
Say you have a production that takes 100 ingots /min. If you choose your largest lines you could split each off, 4 productions with the remainder 76.7 left out. You could merge those together. Then the 450s you do the same, and merge all those 50s in. Then you have one (or two lost count) belts of just those remainders left that you could then maximize production with.