The 100-110% of last year's tax liability is a "safe haven" allowed by the IRS and since it's an exact number that you can look up, that's usually what I recommend. Keep in mind that your tax liability is going to be reflect all your income, not just your self-employed income, but other tax payments you may make (such as withholding) are going to be factored in as well.
The 4.95% is for state income tax. For federal income tax, I still recommend sticking with the 100-110% amount, it's just the simplest way.
Hope this helps!
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