Equivalence of zone-grays to digital grays?

[titrisol]

In order to make a suceesful digital grayscale I'd like to know which are the equivalents of the grayscale/fstops in the digital grayscale (0-256)

0 = black or zone 0
256 = white or zone 10
46 = 18% gray or zone 5 ???

Thanks

[vet173]

0=0,I=26,II=51,III=77,IV=102,V=128,VI=153,VII=179, VIII=204,IX=230,X=255