Definitely different for every cat! Everything is, unfortunately. But in general, you want one in the mid to low yellows, with a nadir in the low blues or high greens.
What you are after is a preshot number that is fairly similar day and night, so that you can continue to dose the same day and night. The trick is finding a dose that gives you enough action in the middle of the cycle, but doesn't make the cycle last too long, or too short, and doesn't cause a drop so low that kitty bounces higher by the end of the cycle. It sure isn't easy to find that dose, as you well know.
The theory (mine) is - get the preshots around 250 or less, and the nadirs about half that. So that the cat is below the renal threshold all day long. When that happens, healing can happen. One of two things happens then. A) You either up the dose just a bit to make the nadir a little bit lower and still not cause bouncing. Or, B) as the healing happens, the numbers at preshot and nadir will come down together on the same dose. If option B happens, then you are able to reduce the dose just a little bit, and the numbers still stay all below 250. You might see a slightly higher and flatter curve, but still all good numbers. Healing continues, and you repeat the process. If all the numbers come down together, you continue to reduce in small increments.
We all say we like to see a nice "smiley faced" curve. But the closer you get to OTJ, the more you want the curve to get flatter, as long as all the numbers keep coming down together. Eventually, you end up with all the numbers under, or close to "normal", at which point you try an OTJ trial.