Multiply your age times your realized pretax annual household income from all sources except inheritances. Divide by ten. This, less any inherited wealth, is what your net worth should be.
To give an example, suppose one is 40 years old and makes $100,000 per year. Then, by the above formula, one's expected net worth is $400,000. The authors then go on to describe the wealthy (prodigious accumulators of wealth, PAWs) as those who have at least double this expected net worth based on age and income. Conversely, one who has accumulated less than half of their expected net worth is known as an under accumulator of wealth, or UAW.
In any case, the suggestion by the two authors is a broad rule of thumb. I would caution one against putting too much weight into the usefulness of a rule of thumb. Nevertheless, armed with this knowledge, you can at least have a rough idea of where you stand with regard to what your savings should be at any point in time.