You do not have to pay those. It is only a convenience - which assumes that this year will look exactly like last year. As long as your withholding gets 90% of the final tax you owe or 100% of the amount owed in the preceding year, there is no penalty. Go to irs.gov and search forms for "penalty calculation" if you like: It gets this:
https://www.irs.gov/pub/irs-pdf/i2210.pdf
