Had not had a traditional IRA until establishing one in early 2016 to fund as a 2015 IRA that would be converted to Roth upon funds arriving into account. MAGI too high to qualify for deductible IRA or direct Roth contributions in 2015 and subsequent years.
Received 1099-Rs for 2016 for those funds and 2016 contributions. Did not file an 8606 in 2015 (mistake on my part?) but did in 2016 and reported basis that did not include funds from 2015 because the TTax question, "...basis as of 12/31/2015" was $0 since funds weren't entered until early 2016 (mistake?) Should that basis be changed on 2015 taxes to reflect contribution amount?
in early 2016 you opened a traditional IRA and funded it with after-tax dollars
in early 2016 you converted that traditional IRA to a Roth IRA
in early 2017 you received a 2016 1099-R for the conversion amount, which you reported including a 8606 in 2016 [edited]
You need to include a 8606 for any year you make non-deductible traditional IRA contributions (to track your basis in Part I) or Roth conversions (Part II). If neither of those things happened in 2015 (or some other conditions (see below) that don't seem to apply), then you didn't need to file a 2015 8606.
The 2016 8606 should show the non-deductible trad ira contributions and should calculate the taxable amount of the conversion by factoring in those amounts.
If you used up all your traditional IRA basis in the conversion you probably won't have to fine an 8606 unless you do certain withdraws, make more non-deductible trad ira contribution, or more conversions.
For the most part TT is using the end of year FMV of IRAs to calculate how much a traditional IRA distribution is taxable and how much is a return of basis (non-deductible contributions). The basis does not come out first (that would be too good), but rather it is $ out x (basis at beginning of year / FMV at beginning of year). So it uses the 12/31 value for that calculation. It also computes the new basis for the next year on the 8606 and uses that in the next year.
For more details see "who must file"
If zero then you paid included too much income for the conversion. Check your conversion year (2016) 8606 part III, line 16 to see if you missed the 2015 contribution basis). In that case I would suggest you amend your 2015 to include the 8606 and amend your 2016 to include the basis carryover (e.g. the right value for 8606 line 1).
But first make sure that gets you a refund for the amended 2016 return (more income does not always result in more tax, it depends amounts, AMT, capital gains vs ordinary income etc.) If not don't bother (assuming you converted all of the your trad ira's and therefore trad ira basis going forward isn't an issue).
It sounds like you understand what is going on here.