Let me qualify my response by saying I'm no expert. Also, I typically find the questions encountered on the state return rather baffling and TT provides little or no explanation. That being said, I can tell you what I just did since it's the only thing that made sense to me.
My son makes an after-tax, non-deductible contribution to his t-IRA every year, then subsequently converts the funds in the t-IRA to Roth (i.e., Backdoor Roth). In such a case, only the gains accumulated in the t-IRA should be taxable for both Fed and State purposes. This amounts to about $2 in his case.
I noticed his MA tax liability was a bit high when I got to this section, so I assumed MA was counting his t-IRA distribution as taxable. I believe the section re
"Other Contributions Previously Taxed by MA"
is supposed to account for the discrepancy and takes into account non-deductible t-IRA contributions. You are correct that these contributions have, in fact, already been taxed in MA as part of your W2 income.
So I plugged in my son's t-IRA contribution made in 2018 into that box and, voila, his MA tax liability declined. I assume it would be OK to add in your 2017 non-deductible after-tax contribution as well.
I'm not sure what to do with the box
"Total Distributions Received in Previous Years." I left it blank. My son obviously receives a distribution every year since he does a Backdoor Roth every year. It doesn't seem like it would make sense to add up all the distributions received in all prior years. That could eventually be a big number (say after 10 years) and I'm not sure what purpose it would serve. I would be interested to know how others treat this section.
By the way, on the Fed side, I learned that when you do a Backdoor Roth and you get to the section where you report the IRA contribution, there is one tricky question I always miss and that results in TT not treating the Roth conversion correctly and higher tax liability. That section asks you to identify any IRA's you "own or contributed to" in 2018. You need to check both t-IRA and Roth IRA. I was only checking t-IRA since that was the only IRA getting a contribution. However, if you also check Roth IRA, then TurboTax will subtract the Roth conversion from the t-IRA distribution.
