Ok. My base and bonus interests do not tally with my calculation for August which I am still checking.
It is the easier path to calculate the total daily interest (3.5%) and sum them up for the month, and then check (prevailing interest credited on 31/8 + bonus interest credited on 20/9) = total interest (3.5%) for Aug
However, if you need to calculate the specific prevailing interest credited on 31/8, there are 2 things to note (for existing customer) :
1) if any of the end-of-day balance from 01Aug to 31Aug is less than $1k (Fastsaver a/c), daily interest = $0 for that day (no need to calculate)
2) for end-of-day balance higher than $1k, then calculate the daily interest for that day using the different tiers (example t&cs: First $25k: 1.50% p.a. Next $25k: 2.50% p.a. Next $25k: 3.50% p.a. Above $75k: 0.80% p.a)
Example: If you have end-of-day balance of $60k on 16 Aug
Daily prevailing interest for 16 Aug = 1.5%/365*$25k + 2.5%/365*$25k + 3.5%/365*$10k
Note: for daily prevailing interest calculation, just use the end-of-day balance for calculation - no need to use incremental balance over the 31July balance (for existing customer). For total (3.5%) daily interest calculation, need to use the incremental balance method.