I just changed my exemptions from 2 Married to 1 Married.

Each withholding allowance on the W-4 is "worth" the individual exemption amount on form 1040. For 2015, that is $4,000.

One then needs to know one's marginal tax bracket to translate exemption to tax. E.g., in the 15% bracket a $4,000 exemption saves $600 and in the 25% bracket that $4,000 saves $1,000.

To increase withholding by $1,900/yr, someone firmly in the 15% bracket would decrease W-4 allowances by $1900/$600 = 3. In the 25% bracket it would be $1900/$1000 = 2. Note that, due to rounding, it won't be an exact change, but close enough....

As the first third of 2015 withholding will have already been done with the existing W-4, a change now would need to decrease by 1900/600*3/2 = 5 or 1900/1000*3/2 = 3 for the above examples.

Also see

http://forum.mrmoneymustache.com/ask-a-mustachian/how-can-we-optimize-our-withholdings/...withholding went down when you switched to Roth contributions.

Not sure why switching to Roth (either from nothing or from traditional) would cause withholding to go down. Seems it would either not change or go up...?

To avoid underpayment penalties you should increase your withholding, by submitting a new W-4 with the same number of exemptions and an "additional withholding" amount of ($1900 / the number of pay periods remaining in the year). That should get you in the right ballpark.

This is a more exact way than the "adjusting the exemption allowances" described above, because you aren't restricted to one significant digit.