It might work a bit better if incomes were bucketed into 5k brackets before taking the mode. From there, I guess you can take the mean or median of all the incomes that comprise the bucket?
Maybe as a rule of thumb it would always be the lowest bracket for the US. I live in the Netherlands, and the mode here is as a rule of thumb about 79% of the average. This gives a mode of 44000 euros annually. So it doesn't have to be the lowest bracket.