Merging belts until fully-loaded and dealing with overflow

Say I have 3 Mk.5 belts transporting the same material at 360/minute. I want to merge these into 2 belts, such that one is fully-loaded with 780/minute and the remaining 300/minute overflow goes off onto the second belt.

I can do this specific example with some load-balancing, but I’m looking for a more generic solution that will work when the numbers aren’t as friendly and load-balancing is too complicated. Is this possible?