It seems to me how the relevant part of the book is the attempt to categorize people in the three (macro) groups, though of course the formula to be used is up to debate.
The 10% of age multiplied by current income sounds a little too simplicistic, as noted in the Wikipedia article it is definitely not accurate for young people, but probably is also not accurate for old/retired people, and it doesn't take into consideration the fact that (probably less likely at the time the book was published) people may have had in their life "bad periods" when they were unemployed or however had much lower than current income.
Is this not obvious to others here?