The other issue is when the taxable basis goes negative. There are two ways that have passed muster. Jrad is absolutely correct in using debt while some just use the capital account number.
If your basis goes negative, in the past, likely that nothing would immediately happen as the IRS did not or could not match individual K1s. That should change shortly. Possibly not for TYear 2020 because of COVID, but pretty sure for next year. Could be ugly for some.
As Jrad said use your K1. It on a sale will USUALLY give the correct numbers. A note that MLPs average all units.
ARB