you want to run it as loose as you can without slipping the belt (which will bog the engine). It will lower your rpms to loosen the secondary, because it will upshift faster, so you may have to remove some weight from the primary. May also have to take out a little helix (run slightly shallower)
It all depends on conditions of the snow or ice. where I live there is alot of snow so when conditions are loose I like to tighten up so that when I spin out of the hole and then hook up I'm not in high gear. the secondary can backshift quicker. On the hard pack you can loosen but you do have to watch for slipping the belt. As for setting it all depends on what secondary spring your running. A secondary spring has to settings side pressure and twist. So in you install a secondary spring with alot of side pressure you may not need as much twist.