Why “Cut Long, Then Trim”
These are the classic formulas every ham handbook uses, such as 468 divided by the frequency in MHz for a half-wave dipole in feet. They already allow for a real wire antenna coming out a few percent shorter than a wavelength in free space.
Your antenna will still differ a little. Insulated wire, the height above ground, nearby trees and gutters, and how the ends are tied off all shift the resonant frequency. So cut each piece a few percent long, put the antenna up where it will live, and measure. If the best match is below the frequency you want, the antenna is too long: shorten it a little at a time. Folding the extra wire back on itself at the ends works while you tune.
End-fed half-wave antennas use a matching unit at the feed point, and the makers of those units often give their own wire length. Use theirs when they do.