As I said, a self-employed retirement contribution is NOT a business expense. It does not go on Schedule C. It is a personal, above-the-line deduction that reduces AGI. It does not reduce net profit.
If marking the Maximize box for a SEP contribution results in a maximum employer contribution of $13,088, apparently your net earnings from self-employment are only $65,440. Net earnings are net profit minus the deductible portion of self-employment taxes. (That would imply that your net profit is somewhere between $65,440 and $70,415, depending on the deductible amount of self-employment taxes.) 20% of $65,440 results in a maximum employer contribution of $13,088.
