I found out the hard way also that all my learning in school (I won't state how long ago) was round down if under $.50 and round up if over $.50, but not correct in the CTR world. Yes, I had to amend, but I had multiple CTRs to fix. My suggestion, possibly wrong, would be if you have one, contact FinCEN and ask. I have found FinCEN to be very helpful. I once had an error with an address on several CTRs and SARs, and was told that since I was aware of the error and it would be corrected immediately, which it was, it was not necessary to amend. Just saying. It never hurts to ask.
For when I am weak, then I am strong. 2Cor 12:10