Reading you post I was wondering where you got your values from. Then I returned to cobrakai's original post... oops! (chair) staying away from the workshop today appears to have been a very wise decision!
So, according to my calculations using the value of 5.4 as explained in the other thread, this would give 5000RPM / 5.4 / 38ipm => 24.4 "ripples per inch" or about 0.040", which is fairly close to the value you estimated.
Here's a quote for Don: Make things as simple as possible, but not simpler.
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