User blog comment:Luxionmk2/Counting BCs.../@comment-75.219.114.240-20140707045429/@comment-24462444-20140707125522

Basically, once the unit reaches max level, it gets a lot cheaper to fuse units to it. The cheapest way to level a units BB is to fuse low level characters to your unit until it reaches level 20. After that, max your unit level, then start fusing characters to it once again.