It seems to me that (1) you need to track the total of your capital balance plus non-recourse debt and (2) reductions in that balance as you receive distributions. For example, if your capital balance plus non-recourse debt (which I will call "the balance") is $9K at the beginning of the year, and you receive four quarterly payments of $6K, the first payment would reduce "the balance" to $3K ($9K - $6K). The second distribution would be split, with $3K going to reduce "the balance" to zero, with the remining $3K receiving capital gains treatment. The remaining two distributions would both be treated as capital gains, as the capital balance plus non-recourse debt would zero when those distributions occurred.
