To size a drawing (for example, from a magazine) from a particular scale to S-scale, use these enlargement or reduction settings on a copying machine:
from O:
reduce by 75%
from HO:
enlarge by 36.1%
from TT:
enlarge by 87.5%
from N:
enlarge by 250%
from Z:
enlarge by 343.75%
To convert from any other scale to S-scale, divide that scale's proportion by 64 and multiply by 100. For example, let's say you want to convert from 1:29 (G gauge) to S. Use the following:
29 / 64 x 100 = 45.3125%
As an aside, Lionel posted a blog on how to measure and determine scale speed for S-and O-scales.