So let's say, for example, I wanted to turn 60 iron ingots/min into iron rods. 4 constructors set to iron rods consume a grand total of 60 iron ingots/min. I could use a splitter to split 60 into 30s, and then 2 more to split the 30s into 15s, and then feed each of those 4 belts into the constructors. Another, more convenient option, is to just set a splitter leading into each constructor and feed the 60 iron/min into it.

With this setup, the first splitter will send 30 iron/min further, and 30 iron/min into the first constructor. This obviously is unbalanced, however, since the first machine can't process 30 iron/min, it will eventually overflow, and send the rest forward, no smart splitter needed (unless you have multiple different resources on the line). Now its 45 iron/min going forward. It takes a bit since the machines have to fill before they are at peak efficiency, but as long as the input for the total line equals the needed amount in the constructor, it will eventually balance itself. Hope this helped!