By November 2017, Facebook announced the Campaign Budget Optimization. A new way to maximize Facebook & Instagram Campaigns and ensure better Return On Ad Spent (ROAS). During the firsts months of 2018, the roll out has been done.
How does the Campaign Budget Optimization work?
When setting up a Campaign, select the "Optimize budget across ad sets" option.
With this option, Facebook will automatically distribute your Budget between all your Ad Sets to get the best results.
Despite the fact Facebook says is the current best way to Optimize your campaigns... doubt it!
As always, test it and analyze it :)