I run my Phoenix slicks untill there are no holes and they still hook the same,, altho this is not reccomended and I do not endorse doing that,, that said,,I would say you could run them untill there is only 10%/15% is all that's left of the hole, unless like Mike says...the sidewalls go away.
I have run Phoenix exclusively and (or Firestones) for over 20 years, the way I do it is when there are more than 4 holes missing I retire them. I carry spares and if I happened to get in a final and couldnt count on the hook due to cord showing I would feel like I made a dumb decision so this is how I do it. I have never experienced sidewalls breaking down on me but the shorter life of Phoenix may be the reason, I get about 120 - 130 passes on a set, I'm kinda hard on them in the burnout box.
hi, short burnouts help make them last longer. toyos, 160 runs still have holes left. not down to the smaller hole yet.this is on a stick shift car. when hole gets shallow or about gone then pull them off.
I've gone into the cords, accidently, more than once The last set was a set of M/T ET Streets that I bought used and loss site of the tread being thin in the middle, saw the cords so off they came. Luckily I carry several sets of rear tires in the trailer now so it meant changing them at te races last month
The holes are just like wear bars on conventional tires. You should be able to run them safely until they are gone - with the same tractability. The compound doesn't change as the tires wear down. It does change with age. The issue is uneven wear, which you have to start looking at when they are about 1/2 gone. Obviously if you have ply's showing, you gotta dump them. One of the reasons I run tubes. I could go ether way with my tires. It's just an added safety margin IMO.