When trying to determine if enough money has been withheld via paycheck or by quarterly payments you need to make sure that either you have pre-calculated the amount you expect to pay, or you have calculated the safe-harbor number. Pre-calculation is hard. Congress has been in the habit of making tax law changes wither late in the year, or even yearly the following year.
The easier goal is to make the safe harbor. For most people/couples the goal is to have 100% of last years taxes withheld. That means you have to know how much were your 2018 taxes. That number isn't what you paid with your return, or how much was refunded with your return. Making the safe-harbor means you won't be penalized, but you still may own a big check in the spring of 2020.
From IRS PUB 505 (2019):
In most cases, you must pay estimated tax for 2019 if both of the
You expect to owe at least $1,000 in tax for 2019, after subtracting your withholding and refundable credits.
You expect your withholding and refundable credits to be less than the smaller of:
a. 90% of the tax to be shown on your 2019 tax return, or
b. 100% of the tax shown on your 2018 tax return. Your 2018 tax return must cover all 12 months.
Note. The percentages in (2a) or (2b) just listed may be different if
you are a farmer, fisherman, or higher income taxpayer. See Special
Rules , later.
Regarding higher income:
Higher Income Taxpayers
If your AGI for 2018 was more than $150,000 ($75,000 if your filing
status for 2019 is married filing a separate return), substitute 110%
for 100% in (2b) under General Rule , later.
For 2018, AGI is the amount shown on Form 1040, line 7.
That means that you can change the W-4 late in the year to make that 100% or 110% level for the year. I haven't gone as extreme as you with shift from 9 to zero, but I have moved it by 3 or 4 before.