If there are more than 64 people fencing in the DEs on the first day, there will be a round of 128. (It is even possible to have more than 128 people fencing the first day DEs, but only at the hugest of WCs, so don't worry about it right now.) Your seed is S. In the round of 128 you fence the person whose seed is 129-S. In the round of 64 you fence the person whose seed is 65-S' (where S' is the same as S unless you upset someone in the round of 128, in which case it's 129-S). Once you make it to the 32, you're in the second day, and you go into the bottom half of the table of 64.
If in our theoretical world no fencer (but you) loses to a fencer ranked worse than he, therefore everyone but you comes out of the pools ordered by their world ranking. This makes it easy to guess at the strength of your opponents in the DEs. Take their seed in the DEs, e.g., 129-S, and add 32 for the people who already made it to the second day. This is their original seed, which you can divide by the attendance to find out their world ranking. And then you can compare their world ranking to your T and find out if in theory you would win the bout.
Second Day Direct Elimination.