in the first mist born trilogy, it mentions that Rashek is a very powerful feruchemist, which has caused a lot of speculation on how feruchemical power is measured, as by its nature, a feruchemists power is determined by how full their metalminds are. my theory is that the relative power of a feruchemist is measured in how good they are at conserving power.
in the end of TFE Sazed states that when a feruchemist taps large amounts from their metal minds that a certain amount of power is wasted. I think that the more powerful a feruchemist is the less power is wasted when tapping large amount of power.
lets say X is the amount of strength required to make a given feruchemist twice as powerful as before, Y is the amount of strength required to make a given feruchemist four times as powerful as before, and Z is the amount of strength required to make a given feruchemist eight times as powerful as before. according to mathematics, 2X = Y and 2Y = Z. In reality, it would be 3X = Y and 5Y =Z. a weaker feruchemist would have 5X = Y while a very powerful feruchemist would have 2.00001X =Y.