Invest in a durometer... its a good tool for figuring the life left in a tire carcass...Good ones get kind of pricey but in my opinion are well worth it....
Not to proud of it but I was running pirelli slick take offs from 07 and 06 last year in my classes.... I have actually never owned a new set of tires(EVER) thanks to ST takeoffs towards the end of the season
Brownie and Casey are 100% correct..its piece of mind as well as your riding pace.
I'm just a cheap ass and still can't afford new tires every weekend..... Hell I ran a set of power ones way down past the wear bars.....almost nothing left on the outside... also the fastest lap times I ever had at HPR :evil: Just goes to show how michelin holds up!
I am not a person to listen to...just a noob....just sharing my experience...