The problem with the 10x of salary is two-fold.

First, and less critical, it needs to be 10x of your final salary, which may be 20x or more of your initial salary.

Second, if your income is high, 10x won't be anywhere near enough. That estimate assumes about 20% expense reduction in retirement, and about 40% replaced by Social Security.

The Social Security Primary Insurance Amount (PIA) is calculated (https://www.ssa.gov/oact/cola/piaformula.html) as 90% of the first $1115 (adjusted annually for inflation) in Average Indexed Monthly Earnings (AIME), plus 32% of the AIME between $1,115 and $6,721, and just 15% of AIME up to the maximum monthly (determined from the maximum payroll-taxable salary).

So, if your AIME is $10,000 ($120k/year), your benefits will be about 33% of your income. Adding the, say, 4% of 10x gives 40% more, so you're at 73% of income rather than the needed 80%.