Thank you for all your help!
FWIW, I'm not afraid of circular calculations. As you know, the IRS instructions have some of that; they end up computing that a 25% maximum contribution by the employer is 20% of the net earnings.
My calculation gives that a 25% contribution is instead 3,717,409/19,717,409 (approximately 18.8534%) of the Schedule C net-profit (as determined before subtracting for the Employer contribution that we are currently computing). With that value then subtracted as a Line 19 expense, then subtracting half of the then computed self-employment tax, and then taking 25% of that ... you get the same Line 19 value back again -- so it is within the 25% limit exactly. (Well, rounding to nearest dollar at various steps might introduce noise. Also, if you max out on FICA or max out on the Employer contribution, those too will change the calculation.)
