Anyhoo, the teacher (back in the 70's) was very specific about which hand/finger went with which finger.
dry run for a bigger attack
The 9/11 conspirators didn't bother with any dry runs and succeeded beyond their wildest dreams.
Terrorists aren't a cautious or methodical bunch; they're audacious. Dry runs are not only for wimps but they also alert your adversary to your intentions.
If mathematically you end up with the wrong answer, try multiplying by the page number.