You have paid all scheduled instalments. Nevertheless, the current loan statement shows a different outstanding balance than in your spreadsheet. For a good query you do not need an invented interest rate. You need the same reference date and a traceable route from a confirmed opening balance to the new figure.
Compare two balances on the same date
First note the date to which each amount refers. A year‑end balance and a balance after the next instalment cannot be compared directly. Also check whether your spreadsheet shows an expected development or already posted transactions. Mark later postings separately instead of retroactively inserting them into the earlier row.
For the reconciliation secure the original payment schedule, the statements for the period in question and confirmed amendments. Do not change the old spreadsheet at first. Otherwise the indication of which assumption led to the discrepancy will disappear.
Calculate with a short bridge
In the entirely fictitious case the confirmed opening balance is 180.000 euro. The statement shows 1.800 euro interest charges and 4.800 euro credited payments in the period considered. If there are no further postings, the balance bridge is 180.000 plus 1.800 minus 4.800 equals 177.000 euro. This is not a calculation of a real loan contract, but the check of these explicitly given postings.
Your spreadsheet gives 176.700 euro. That leaves 300 euro to be explained. Now search specifically for a differently treated payment, a deviating period or an additional item. Use the actually shown interest postings at first. Your own interest formula would introduce a second unexplained basis of calculation.
Tailor the bank query to the difference
A useful enquiry reads: “For the period from A to B I arrive at 177.000 euro based on these postings. My earlier plan gives 176.700 euro. Which assumption or posting explains the 300 euro?” Attach the affected rows, not an unnecessary complete collection of private documents.
If an item is unclear, have the amount, booking date and effect on the debt balance explained. An unexplained difference does not yet prove a bank error. Nor does a blanket description such as rounding make the difference comprehensible.
Only continue after clarification
Add a separate correction row with explanation and supporting date in the plan. If the matter remains disputed, document the open difference and obtain appropriate advice. The work is successful if you can explain the current balance or name the precisely missing explanation. An apparently matching spreadsheet without verifiable transitions does not achieve this goal.