@elgato81 you can simply cross out the $2300 and use $1500, or whatever number you prefer.
Unless you tell the IRS differently (by completing form 2210ai), the IRS presumes your income (and the related withholdings) are received evenly throughout the year and therefore, any tax due is due throughout the year, which means quarterly.
It can be a complicated topic and look at form 2210 for more informaetion.
https://www.irs.gov/pub/irs-pdf/f2210.pdf
p.s. how could the 20% withholding cover "most of the liability": if you owed over $6,000? it may cover most of the liability on the 401k withdrawal, but not the total liability due to the IRS.
