Robert Conrad, Registered Piano Tuner/Technician, Tucson, AZ
The prime 5ths are used to determine the best prime octave width for the particular piano being mapped and eventually tuned. When the widths of the prime 5ths are added together, when that total is -3.0, the prime octave width is about right.
For years and years, I have used the triple octave location for A7. It’s not a bad spot either. But since March 2020, I’ve begun tuning A7 higher than that. I’ve started using the triple octave +5th as a starting point for my A7 location.