I don't know the answer to your question, but Aida Favia-Artsay's Caruso book would be a good place to get the numbers for several standard Victor speeds. And, 75.00 is the one that she says is the same for both line frequencies. From looking at her other examples, though, it's obvious that the conversion is non-linear.
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The Crane School of Music
SUNY at Potsdam, NY 13676
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From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Steve Smolian
Sent: Sunday, January 20, 2019 4:55 PM
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Subject: [ARSCLIST] Playing speed
Is there a formula to covert a specified playing speed at 50 cycles to the
playing speed at 60 cycles? Example: 75 rpm at 50 cycles, what speed at 60