The ratio of the clock is determined by the length of the operating session you want. On my layout, I run from 6am to 6 pm, or 12 hours. I run the clock at a 4:1 ratio, giving me a session length of 3 hours.
This holds true, whether I model in HO or N or any other scale.
There is a group at Yahoo called the Ry-ops-industrialSIG. If you join it, and go to the Files section, you will find a couple of programs for fast clocks, that you can use on your computer. I use one, and it has made all the difference in my railroad's operation. HTH,
Merry Christmas, Bill Donahue