Estimated Tax payment amounts for 2018 now appear to be substantially insufficient. Should I make up the difference now or add the additional amount with the Jan payment?

Paying in additional estimated tax payments now will reduce any potential interest charged by the IRS.

You also should look at your 2017 tax liability.  One exception to any interest charge on underpayment of estimated payments is as follows:

• If your AGI is less than $150,000 and you pay in 100% prior year tax in equal installments (or through withholdings or combination)
• If your AGI is more than $150,000 then you would need to pay in 110% of prior year tax as noted in bullet #1

You can pay directly at <a rel="nofollow" target="_blank" href="https://www.irs.gov/payments">https://www.irs.gov/payments</a>