Form 8606 requires you to compute, each year that it changes, your basis of 'nondeductible' (really, nondeducted) contributions, if any, in your traditional IRA. The change can be an addition (due to a nondeducted contribution), a subtraction (due to a distribution or conversion 'using up' some or all of the basis), or both. The 'backdoor' procedure you followed resulted in both an addition and subtraction each year leaving the result at zero.
Form 8606 does not require you to report basis (of contributions and conversions) in your Roth IRA. You should keep a record of this (preferably with your tax records, so you can find it when needed) because you may need it in the future to determine -- and if audited to prove -- whether distributions you take are taxable or not. But you don't report Roth basis on 8606 or anywhere else; you only characterize and report the distributions when taken, which you haven't yet.
I thought my answer in the Q you linked summarized the definition pretty well:
Thus your basis at any time in a Roth IRA is the total contributions/conversions i.e. post-tax money put in, less any portion you've already 'used up' in previous years
Conversions are post-tax in the Roth, whether or not they were post-tax in the traditional, because if they were pre-tax in the traditional they are taxed at the time of conversion. The 'backdoor' ideally avoids having any pre-tax money in the conversion, and thus any tax at the time of conversion.
ADDED: as far as distributions, the simpler case is the design case that you take 'qualified' distributions: after the Roth account is open at least 5 years and you are 59.5, disabled, dead, or certain 'exception' cases like first-time homebuyer or excess medical. In that case, you don't use 8606 part III at all; you simply report the distribution as nontaxable on your 1040 (formerly line 15a but not 15b; now 4a but not 4b).
The IRS receives information about your contributions and conversions each year on form 5498, and distributions on form 1099R, from the custodian/trustee, and you receive copies. In addition you showed conversions on your return in 8606 part II, and in this (backdoor) case the nondeducted traditional contributions in part I. The custodian/trustee can distinguish most factors for 'qualified' and puts them in the code in 1099R box 7; see the instructions for that (available on the IRS website). This means in many cases where you claim a distribution is qualified, IRS can verify it from information they already have; in other cases if they suspect you are cheating, they can ask for additional information on the specific exception you claim, which is simple (at least on the IRS scale).
In the more complicated case where you withdraw early (and without an exception), you need records of your contributions and conversions and to what extent previous distributions used up basis. This is done on 8606 part III whose instructions are here -- the version on the website is still 2017 as of now, but I don't expect this area to change significantly for 2018 except in the line references to 1040. The instructions (and form) are also available in PDF, if you prefer that (as I do); look under the 'publications' section of the website.
You start with your nonqualified distributions on lines 19-21. 19 includes first-time homebuyer and disaster distributions but the former are subtracted on 20-21 and the latter handled later. (Don't be misled by the item excluding 'return of contributions'; that means a distribution to remove excess contributions in the same year or early enough the following year. It doesn't mean the normal case where you put in contributions, invest them, and later take distributions that are treated as coming first from the contributions.)
On 22-25 you effectively subtract your basis. This is done in two chunks apparently because of the interaction with form 5329, but if you read both parts of the instructions together, they add up to:
if you have not previously taken any (Roth) distribution, just add up all your contributions and conversions. It doesn't tell you how to do contributions, but presumably saving copies of (all) your prior years 5498(s), or the information on them, is good. For conversions it says to use all your prior years 8606 part II, unless you have no conversions at all in which case the conversion basis is zero (but that's not your case).
if you have previously taken a distribution, it tells you to use the worksheet and table which appear about a screenful below the instructions for line 25b. These effectively tell you to start with the then-remaining basis computed on your most recent 8606 part III, and add to it contributions (similarly unspecified) and conversions (similarly from past-year 8606 part II) in all years since (if any).
You subtract your basis from the nonqualified distributions, make a further exclusion for disaster distributions, and if there's anything left that goes on your 1040 as taxable income. And as hinted, it may also go on 5329 for the additional 10% 'penalty' tax; you need to follow separate instructions for that.
In short, stick with the simpler case if you can :-)
