Golden rule with fronts: When in doubt, change it out. So what if a front tire costs $200 new and you throw it away early? One might be able to argue that you could have done another 3-5 races on it, or another track day, 2 track days...whatever. So you "throw away" $60, $80, whatever, of "tire life" left in it. All it takes is one front end tuck and all of a sudden you're spending a hell of a lot more than a front tire.
It took me years to learn and feel when a front is going off. Years. Lots of laps. In short, the front will start to profile like a rear does when you go too long without flipping it, and the bike won't want to finish the corner because the profile. It's just much harder, if not impossible to see on a front tire because it's so much skinnier than a rear, so it's more a feel than visual. And no, don't ever think about flipping a front tire. If you flip front tires you probably flip condoms too.
I've learned to feel when the tires degrade in grip to a certain pace. For the front, I can feel the bikes resistance to finish a corner. And it feels much different under braking/trail braking. Too many times my friends and I will say "I got shit tires, I'm only good for 17s/18s/19s unless I swap out the front/rear"
But the grip demands are so vastly different. A worn front tire that's struggling to do 15s/16s at Summit will be able to do another 70 laps doing 18s. And after you did those 70 additional laps running 18's, it could do another 100 laps control riding. As long as the cords aren't showing, just about every N2 coach out there can coach with them.
So if you're racing and pushing for the podium, you will absolutely have a "worn out" tire that's no good to you but another rider might be about to get another 3 sessions, maybe whole track days even.
Again, situation dictates.
