What IRS tells you is, 

"There are no specific guidelines for reasonable compensation in the Code or the Regulations. The various courts that have ruled on this issue have based their determinations on the facts and circumstances of each case."

 https://www.irs.gov/pub/irs-news/fs-08-25.pdf

What IRS doesn't tell you is that there are very few court cases, and most of those are where owner/employees took zero salary.  And despite the saber rattling about what they are going to do with an extra $88 billion from Congress, you have to remember that Congress has since taken away $87 billion.  Or something like that. 

It's logical to assume that the greater percentage of funds going to owner/employees are considered salary, the less likely an audit.  But of course, little about taxes and audits is logical, and nothing can be assumed. 

ADP has an interesting suggestion:

"The 60/40 rule is a simple approach that helps S corporation owners determine a reasonable salary for themselves. Using this formula, they divide their business income into two parts, with 60% designated as salary and 40% paid as shareholder distributions."

https://www.adp.com/resources/articles-and-insights/articles/s/s-corp-payroll.aspx#  

Of course that has nothing to do with looking at what non-owners earn in similar jobs in similar locations.  It is all about, "What can I get away with?"  So you need to figure out, that's really the basis of any discussion on this subject. 

And the court cases tell you only what happened when IRS and the taxpayers did not settle before trial, as happens in a high percentage of cases.  What kind of case doesn't settle? The most recent Tax Court case I found on "reasonable compensation (Blossom Day Care) does not even involve an S corporation.  The taxpayers were paying personal expenses and deducting them on their 1120.  IRS disallowed the deductions, but reclassified them as wages, so they could collect payroll taxes.  The case was decided in 2021, but the years involved were 2005 through 2008.  IRS might have enough auditors to begin "reasonable compensation" cases, but the Tax Court has been down a couple judges for years, and has better cases to decide than those that are heavy on "facts and circumstances."  And IRS knows this.  

See also the MacAlary case, decided in 2013 by Judge Guy, involving a real estate broker in Southern California in 2006.  IRS had an expert witness (an IRS employee) who said reasonable compensation was $101,000.  The taxpayer's preparer had reported zero wages on the 1120-S, and did not prepare any payroll tax returns or W-2 forms.  The judge looked at the report and said $80,000 was more like it.  The taxpayer had to pay failure-to-file penalties, which would have been avoided even if minimal amounts had been reported quarterly.