Hmm.. There are two discrete forms of wealth.
1) capital liquidity based wealth (eg, what grows without limit in the banking system. Amusingly, this also covers owning other people's debts.)
2) Physical commodities in excess of those required to stay alive or perform a vocation. (You know, things like yachts and caviar, pearls, diamonds, and the luxury of a full hired domestic staff for your 20+ mansions. Also all those rental properties that Orange Idiot is so fond of.)
I would propose a 4-quadrant assay to determine a "real wealth" value, based on the current "lowest poverty extant" baseline.
Actually impoverished or low-income people will naturally also have fewer luxury goods (careful, some expensive items can be manipulated to be luxuries, but they might not be. For instance, an entry level coder, or even a college student with crippling debts, might own some high powered computing equipment. Care needs to be taken to ensure that "neededness" covers vocational requirements as well as baseline existence/viability requirements (food, shelter, medical care, et al). Likewise, they will also have the least total capital liquidity. (nearly all of their wealth is tied up in physical objects; many struggle to own their own home, and it is a major investment for them.
We could take the lowest value holder, and put them at the zero point at the bottom left of a 2-axis graph, with "% physical commodities owned as luxuries" as the vertical, and "% of capital as free capital" as the horizontal axis.
We could then grade and plot every citizen on where they sit on this metric, with absolute distance from the zero point being what determines your "Wealthiness" score. The absolute wealthiest would score very high on both metrics, and would be in the far right upper corner of the scatter.
Armed with such a "floating unit" "wealthiness" value to grade on, we can construct a coherent progressive tax.
Hiding investments or ownership of properties (My 1mo old son owns all those properties Your Honor! Honest!), or trying to disguise capital wealth as debts (Got 30 million dollars? Want a sweet deal? Come to my bank, and I will lend you 60 million, on condition you make a non-interest rated long term deposit of that 30 million, and keep it there for 10 years! WOW, look how quickly you just turned 30 million into a 30 million debt! Of course, you play golf with the board of directors for a major bank, because you are fucking loaded, and so can get such absurd deals quite easily, but what's a little fiscal manipulation between friends?) would need to carry some pretty intense punishments of course.