Under Rule 31D, the RSP/MRP is treated as a cum-tax value and not the taxable value itself. So for GST reporting in GSTR-1 and GSTR-3B, you should reverse-calculate the taxable value from the RSP (e.g., divide by 1.28 for 28% slab) and then report that computed value along with the corresponding tax. This approach aligns with how valuation under Rule 31D works and is consistent with what experienced practitioners have pointed out here.
Since your actual invoice values (IGST vs CGST + SGST splits) will differ from the reverse-calculated taxable value, you can show a discount or adjustment in the e-invoice so that the total consideration on the invoice matches the price received, while your returns reflect the correct taxable value. That helps avoid mismatches between e-invoicing and return filings.
If you need precise numbers for your specific case, it's best to apply the reverse calculation on each sale and ensure consistency between invoice and return reporting.