As a followup to my last post, here is the spreadsheet I use to calculate how much interest I've accrued based on when my payment will be applied. Remember to add in 2-3 business days from when you actually submit your payment online until it goes through.
Put your interest rate in C4. Put the amount of principal you owe after your last payment into cell C5, then put the date your last payment was applied to your account. Once entered, the table below will calculate how much interest has accrued on each date for the next month and half.
For example: Say you have a loan at 6.55% interest, you statement comes on the 26th, and due date is the following 13th, and your minimum payment is $400. Say your last payment was made on 5/26, it gets applied on 5/28, and that payment lowers your principal to $50,200.
Now you'll know that if you make your next payment on 6/26 when you get your next statement, it would be applied on 6/28, and ~$280 of your payment will go to interst. If, however,you wait to make your payment on 7/11, it gets applied on 7/13, then that same payment will have $416 dollars going towards interest. If you paid the minimum, your principal would actually go up by about $16.
Of course, your next months payment will only start accruing from the 13th, so you could make up some of it the next month, but it's just good to know exactly where you stand.
This spreadsheet is not exact, but it gets within a couple pennies of what Citibank really charges me. If anyone has any ideas on how to make it better, please leave a comment.