Even if you had a "blunder" last month, if I'm understanding correctly... you just recently paid off the existing balance (down to $0) which would have nullified any possibility of interest being charged.
For instance, if you charge $500 and you end up with a statement balance of $500 on the 17th when it cuts, you have until the 12th (estimating) of next month to pay off that statement balance. If you make a payment(s) totaling at least the statement balance by the due date (12th) you don't pay any interest. on those charges.
So, let's say you had a statement balance of $500, paid that total amount by the due date, but charged $650 between now and then... you will have a new statement balance of $650 on the 17th of next month with another 25 days to pay that amount before accruing interest.
The grace period is basically the date between the statement cut date, and the payment due date. (17th statement cut date, and 12th payment due date of the following month). I'm unsure of what the exact circumstance was with the previous interest charge, but you should not be charged any interest as you already paid the statement balance (and then some, likely). That $400 should be safe.
If you did in fact pay down to $0 recently, you should be safe charging however much you want to without worrying about the statement balance, as you have until the following due date to avoid interest charges.
If that is unclear, feel free to ask more questions. I remember when I wasn't totally understanding of cc's and thought that I was going to be charged interest instantly on any statement balance that was cut on that same date, so I paid everything the moment it posted. That of course is a good mentality to have, but isn't necessary to avoid interest.
