Conversions as recharacterizations are two different things. If you did a recharacterization of a traditional IRA contribution to be a Roth IRA contribution instead, the addition to your basis in Roth IRA contributions would be the amount of your contribution, the amount originally contributed as a traditional IRA contribution.
However, it sounds like you did a Roth conversion, not a recharacterization. The Roth conversion would result in basis in Roth conversions, not basis in Roth IRA contributions.
It's also not clear what a Roth IRA contribution for 2018 has to do with any of this. Form 8606 is not used to report Roth IRA contributions other than Roth conversions.
