The inner and outer temperatures must differ between 6 and 10 degrees, with the inner being the hotter of the two. This is the rule of thumb I use anyway. Maybe the rears a bit less difference - say about 5 to 8 degrees. This is mostly affected by adjusting the camber. If the difference is too little, add more negative camber, if it's too much, decrease negative camber.
Tire pressure is what affects the middle part - the crown. If you have the above more or less correct, the middle temperature must be as close to in the middle of the outer 2 as possible. This means that you have as much of the entire contact patch on the road as possible, as much of the lap as possible.
For example, say the optimal temperature for a tire is 90, as above. That means the front must be at about:
- Front: 86 OUTER 90 MIDDLE 94 INNER
- Rear: 87 OUTER 90 MIDDLE 93 INNER
Now, obviously this changes a lot over a single lap, but the aim is to get it as close as possible to these values in places where you require the most lateral grip, i.e. fast, long corners - Pouhon, Istanbul Turn 8, etc.
Obviously everything is a compromise, as some values might decrease grip under acceleration out of slow corners, but as close as possible to these as you can get, is optimum.
Correct me if I'm wrong, please, as I'm still learning a lot about setups, but this is how I understand it.