Basis is the amount of nondeductible contributions you have in your traditional IRAs. Your basis nondeductible traditional IRA contributions, in this case $6,000, is rationed out in your distributions in proportion to the overall value of your traditional IRAs. That ration for your $6,000 distribution form the traditional IRA valued at $101,000 after the contribution would be $356.44.
The "backdoor Roth" method only works as desired if you have no other money in traditional IRAs.
