So you don't think inflation adjusted spending power defines class? Income percentiles can't really define progress as of course there will always be e.g. 20%

So you don't think inflation adjusted spending power defines class? Income percentiles can't really define progress as of course there will always be e.g. 20% in the bottom 20% of society and their relative income to the top is meaningless as it could just as well mean that the top of society has nothing as it could that the bottom of society has lots. So you need to measure spending power, which is exactly what this paper did.