I know the rule of thumb is ten but I've never went that far, always seemed to rich for my combo, aren't you bracket racing this thing?
If so your gonna want it on the lean side anyway, but not too lean. you'll have to read your plugs to determine that.
A leaner mixture wont pick up ( e.t. ) as much when it cools off later in the night. you'll run much more consistent that way.
Goodluck, Charles