How do the Spark Tables work?

[ferocity02]

I apologize for the noob question, but could someone explain how the spark tables work? There is a high-octane table, low-octane table, optimal spark table, and base spark table, and maybe more. At highway speed at lowish RPM and moderate TP I am getting some knock retard and I'm confused as to which table I need to be looking at. Also, during normal cruising and fully warm, my spark does not match any of the tables. It's close to the high-octane table but usually 1-2 deg below. Could be due to interpolation I suppose.

When knock retard occurs, how is the new spark calculated?

Feel free to post a link, I couldn't find one :doh:

Thanks!