Average rating axis "how-to" please ?

I have 4 axis voting system with fivestar as cck fields. I want to order nodes by overall rating.
If if understand it correctly, 'vote' axis does not calculate automatically out of the box, so I need to calculate additional average 'vote' axis after submitting (or updating) review node (it will just use average of 4 axis values).

What would be the best place to do it ? If it's hook_nodeapi, what 'op' is the best ? or maybe use some hook of votingapi itself ?