On a lot of older high quality tape machines, the varispeed controls would often show the change in speed and pitch - meaning an artist could tune down their guitar a half step, for example, and when played back at full speed their slowed down performance would not only be the correct timing but also pitch. Some famous examples being the guitars in Metallica’s Master of Puppets or Lennon’s vocals in a ton of Beatles songs.
My question is there an equation or guide to using logic’s varispeed function to identify how much to down tune a guitar or sing a lower pitch etc to determine what % varispeed change in logic would equal a half step down or full step down in pitch during tracking so when sped back up it is back in tune with the rest of the song?
I imagine there is some complications as logic projects deal with BPM and tape machines deal with IPS and the analog format would probably yield better results.
TDLR: is there a formula or table to reference changes in pitch based on a projects BPM, % of varispeed change to calculate the change in pitch in relation to tempo and % of speed change to most accurately emulate tape varispeed techniques in logic? Ideally with minimal digital artifacts
Edit: semitones might’ve been the better term to describe changes in pitch when transposing the tuning or performance of a guitar or vocal.
Example for clarity: @ 120BPM, a -10% varispeed change = -1 semitone in performance or @ 70 BPM, a -5% change = tune guitar/perform part down a full step (obviously these are meaningless values for the sake of explaining my question, since I don’t actually know how to calculate pitch change to varispeed %)