I was scrutinizing my statements recently, and a useful thing I found was that you can get a lot more detail by creating a flex query. You can set it to show things like the exact balance used for each interest rate tier and the collateral posted for short positions. On most days I could exactly calculate the accrued interest, but sometime there was a discrepancy that the flex query resolved. In my case, sometimes they would slightly tweak the interest rate for a prior day or make some other adjustment, and the difference would be posted to the current day's statement. Also, interest on short sale proceeds seems to only be applied sporadically and then several days' worth gets posted all at once.
Thanks Elt, will have a look at Flex queries. Fwiw taking the initial requirements of futures and Fops still works as a good approximation in my case, just checked again today. Might not work as well though depending on one's portfolio.
When you talk to IB, how do they provide you an answer (a spreadsheet or)? Seems impossible verbally.