There is one MOW plan (#115.4), which lists individual spacings from 18" to 23.5". However, while I could confirm the geometry of that plan, the ties are wrong, because that plan states 27 ties from headblock to point of frog.
Reality shows 29, sometimes 28 and that covers IMO 90% of the cases. Also the tie spacing varies enourmously, especially turnouts with 4 switch rods seem to have enourmous gaps sometimes.
With 29 ties I have an average of about 20.25", but that's really just an average.
Christian