It is a magic ratio. The hub is just a Shimano 105 cassette hub.Is that one of those White Industry Eno eccentric hubs with the offset axle that allows for adjusting the chain slack on bikes with vertical dropouts? Or did you just get lucky on the chain length. I've done many single speed conversions and a couple with vertical dropouts. On those I kept trying different chain ring - cog combinations to get the chain slack right and still have a reasonable gear ratio. I've never like the singulators as they can throw the chain.