Dizzy
You are correct....whatever the distance from the PAP to cause inverted flaring or back ward flaring, would be a negative pin. And that is not always 6 3/4" from the PAP. It is whatever point the pin from the PAP causes the track to stop flaring one way and invert. Could be 6", 6 1/4", etc.
I have laid out balls with pins from the NAP...full rollers as well as the 'boomer' layout.
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Formerly BrunsRico