[78-L] Calculating 78 speeds in percentages

[Sammy Jones]

A math-challenged 78 collector needs your help.  When using a turntable with
a digital read-out for the pitch shifter (like a Technics SP-15), how do you
calculate the percentage to get from 78.26 to other common "78" speeds like
76.59, 75, 74, 80, etc.?  There's got to be a simple formula or equation,
but my high school algebra is not coming back (even if I had it in the first
place!).

Similarly, there should be a way to determine the playback speed in RPM from
a record I've pitched by ear that, say, plays back at 4% below 78.26.

Many, many thanks...

Sammy Jones