You have to look at all of your income and tax withholding from other sources, plus any amount of self-employment earnings in excess of your Solo 401(k) contributions and the deductible portion of self-employment taxes and base your need to make estimated taxes on that. The estimated tax payment would generally be only for your self-employment tax liability only if all of your net earnings from self-employment went to your deductible Solo 401(k) contributions and the deductible portion of self-employment taxes (the sum of which cannot exceed net earnings from self employment (and your other sources of income had sufficient tax withholding to cover the taxable income in excess of your standard or itemized deductions).
Another way to say it: If without the self-employment income you would not need to make an estimated tax payment, and all of your self-employment income goes toward your Solo 401(k) contributions and the deductible portion of self-employment taxes, your estimated tax payment would need to cover only your self-employment taxes.
You can use Intuit's TaxCaster to estimate your overall tax liability. This uses 2019 tax tables but the result should be close: https://turbotax.intuit.com/tax-tools/calculators/taxcaster/
