ANSWER:

This concept is discussed extensively in Mastering Elliott Wave. Precise Complexity measurement is part of NEoWave theory, but not part of orthodox Elliott Wave. 

Wave-a of a Flat is the start of a new trend, so it tends to be violent. Wave-b of a Flat is a “correction” of that new trend. Corrections, as a rule, are nearly always more complex and time consuming than the trending segments of a pattern (except in Terminal formation). This is especially true when comparing the time and complexity of a correction going with the trend as opposed to one going against the trend. Consequently, the a-wave of a Flat must always take less time than wave-b. Since a crucial aspect of time is complexity development, that makes it impossible for wave-b to subdivide less than wave-a in a Flat. If you see wave-b take less time and complexity than wave-a, a Triangle, Diametric or Symmetrical pattern is forming, not a Flat. 

Wave-c in a Flat will take either 50% of the total time and complexity consumed by waves-a and b combined or it will take approximately 100$ of the time and complexity of waves-a and b combined.