Your calculation for bonus interest is not entirely correct.
Month of October doesn't get any bonus interest, because that is the month the account was opened. Look at the FAQ's calculations again carefully.
Bonus interest is current counter * lowest balance of the *previous month*. You generally need to shuffle the 'bonus interest' one column down.
01 Oct 2018: 15,000.00
01 Nov 2018: 15,010.19; 15,000.00(Oct balance)+10.19(base Oct)
01 Dec 2018: 15,021.31; 15,010.19(Nov balance)+9.86(base Nov)+1.27(bonus Oct)
01 Jan 2019: 15,033.98; 15,021.31(Dec balance)+10.20(base Dec)+2.46(bonus Nov)
and so on.
(Also, assuming you opened and funded the a/c on 1 Oct. If you did it on, say, 21 Oct, then base Oct would only be 10/365 days only)
But ultimately, such detailed calculations are pretty meaningless, unless you plan to track the daily sibor rate. Interest is calculated *daily* (against the daily sibor rate) and credited monthly; and chances are, the calculation is rounded down *daily* instead of monthly, so that could make a pretty substantial difference if you're estimating it monthly.On top of that, interest only posts in the account in the next month, although it will be dated the last day of the previous month.