The 1600 referred to is the starting total length of the the forty 40" strips. Out of the package. But the 40" is length isn't fixed as the WOF does vary among fabrics (and jelly rolls).
You don't need to make any adjustments or add any more length for the quilt to work and finish as seen in the video (and everywhere else).
So if the kids/you are cutting from stash, then all you need to start is the forty WOF (width of fabric) strips.