The calculation is correct. According to the IRS, the deduction limit for self-employed individuals is first reduced by the deductible half of the self-employment tax and also by the SEP contribution before getting to the income amount that is multiplied by 25% to determine your maximum contribution.
If you made $100,000 from self-employment, less your deductible half of self-employment tax leaves you with $92,350. Now you have to do some circular math to determine your maximum deduction, but it turns out to be about $18,470 maximum deduction leaves your deduction limited to 25% of $73,890.
If you take it from the number you start with, as @fanfare stated, it is about 18.47% of your net income before any deductions. This is your SEP limit for self-employed individuals.
@lmarsha1
