"There are a significant number of variables"
I used specific language, including "Profits", which in IRS-speak means bottom line on the Schedule C. My statements are accurate as stated; obviously "variables" will alter the outcome, just like with any tax return.
"...probably make one's effective tax rate considerable lower than that"
First, I was talking specific dollars: one owes the Feds over $10K, the other not even 10 cents. Second, effective rate is almost always lower than marginal rate, except within tax pockets*, or in a flat tax system.
*A "tax pocket" is an odd pocket in the tax code, when one particular move, often an IRA contribution, turns another dial in the equation, and the result offers an outsized bang for the buck. For example, sometimes a taxpayer can be in a 25% marginal rate, but $1000 into an IRA nets them $450 in refunds.
In extreme cases, one dollar can trigger thousands and thousands of dollars in savings. My record is one dollar triggering $10,578, when a pair of idiot clients who shouldn't have been taking a subsidy, finally put enough money into their IRAs to get down below the "pay every dime back" level.
The most common tax pocket is the 434th dollar of Self-Employed Profits on a Schedule C. Filers suddenly owe $60 more in federal taxes on that one additional dollar of income, for a 6000% marginal rate