Both $9777 and $10000 give 67 cents per day.
The $223 difference could be earning interest elsewhere.
Correction: Maybe some days earn 68 cents, I guess there's some rounding involved? Will do further testing
I have put 2 days
Day 1 with $1000 - total interest $0.06
Day 2 with $9777 - total interest $0.72 (increase of $0.66)
Counting backwards, I suspect the formula is deposit * 2.5/100 / total calendar days in year.
Applying formula:
Day 1 - $1000 * 2.5/100 / 366 = $0.068 but rounded down to $0.06 instead.
Day 2 - $9777 * 2.5/100 / 366 = $0.667 but rounded down to $0.66 instead.
Therefore, using same formula, the ideal deposit amount to get same interest as $10,000 ($0.683/day) is $9955.20 (exactly $0.68).
Going to test this out for Day 3. Haha.