Late GSTR-3B: is interest under section 50 computed on net cash liability, and do we pay it in the next 3B or via DRC-03?

A client filed GSTR-3B for July 2026 three days after the due date. The late fee was auto-populated in the return, so that part is settled. For interest, the client had input tax credit covering most of the output tax and paid only about Rs 1.8 lakh through the electronic cash ledger. My reading of the proviso to section 50(1) is that interest is charged only on the portion of the tax paid through the cash ledger, and that this applies retrospectively from 1 July 2017. Two things I want to confirm from members who handle this regularly: 1. The interest that the portal shows in the next month's GSTR-3B (Table 5.1) — is it computed on the net cash portion only, or should we recompute and edit it before filing? 2. Is it fine to discharge the interest through the next GSTR-3B, or is DRC-03 the cleaner route so that a later mismatch notice does not come up? If anyone has had a notice on exactly this point, I would like to know how it was closed.