Here's what it sounds like is going on. . .
You knit the back to, let's pretend, 22", and then begin the shoulder shaping. But on the front, you have to start the scoop of the neck lower down than the shoulder shaping. So instead of going to my imaginary length of 22", you have to start the scoop at, again just to make up a number, at 18". So that's where the 24 fewer rows comes in: Let's pretend you're getting 6 rows to the inch. So if it took 132 rows to get the 22", you're going to need to stop at row 108, that is, 24 rows earlier, to start the neck scoop shaping on the front. Make sense?
Here's the one place you might run into trouble-- if you've changed yarn, you might be getting a different vertical gauge than Harding was using. If the pattern assumes my made up number, for example, of 6 rows per inch, and you're getting 5 rows per inch, you would need to stop 20 rows earlier, rather than the 24. Keep in mind all of my numbers are imaginary!-- you'll need to make the adjustments based on the actual gauge and depth of the neck scoop in the pattern, and not my pretend figures!