I think leaving two days on either side is just being safe. It really only has to be one day left, but it's easy to screw it up, depending upon your reserve period. People don't think about when the reserve period ends or starts, and putting in the 12 hour buffer. It definitely requires some more careful thinking, but I am dead sure that if you account for when your reserve period actually starts or ends, add that 12 hour buffer, and if you do it correctly and there is one day on either side, it drops the entire block.
However, it is easy to screw up if you don't think carefully about it. Particularly if you have an R24 period.