You generally minimize your taxes by maximizing your deductions, so I don't see the difference unless you have a large amount of other deductions such that your taxable income is reduced to zero before you reach the maximum solo 401(k) contribution.
"I would still like to know the answer to the "25% times what ""
I answered that. It's 20%, not 25%, times net earnings. Net earnings are net profit minus the deductible portion of self-employment taxes. The 25% factor applies to W-2 employees, not the self-employed who must use an adjusted factor.
If you are now getting a different number for your maximum contribution, perhaps you subsequently entered business deductions that have reduced your net profit. Check your net profit on Schedule C and the calculations on the Keogh, SEP and SIMPLE Contribution Worksheet. (I assume that you do not have any W-2 income that could result in reaching the limit on Social Security wages which could mean a slightly higher employer contribution due to a lower deductible amount of self-employment taxes.)
