It’s sort of too late already, but in case I need to do something like this in the future:
The back of an X-Acto is not a bone folder. Use a proper bone folder. Even the back of an X-Acto is still too hard; it applies too much pressure and weakens the fold. Having to use the mending tape drastically slows you down, because it’s so hard to take the backing sheet off the tape.
Scoring before folding might make sense if you’re working with card stock, but scoring paper weakens it too much. Just fold it carefully at the corner, then use the bone folder.
Did I say use the bone folder? It’s essential.