One way to think of it is in percentages. You aren't looking at absolute #s between PS and nadir, you are looking at the % change. You want something in the 65% range, give or take maybe 5%, and from what I have seen that seems to work well with PZI. Steeper is usually too steep, shallower usually doesn't get them in good enough #s, with some exceptions of kitties who are very well regulated and don't zoom much.
Another way to think of it, following on what Nancy said, is that their #s often zoom once the PZI poops out. If you look at data at +1 and +2 that can give you a sense of their zoom rate, and that is what you are shooting into so to speak. So if you have for instance:
AMPS 350 3u
+1 400
+2 450 --> shot starts kicking in here
+6 150
etc
vs:
AMPS 200 3u
+1 300
+2 450 --> shot starts kicking in
+6 150
you can kind of see how it might work. The shallow curve in the second example is an illusion really b/c of the zoom. Of course it isn't that straightforward, and zoom rates can be variable and hard to predict, and the zoom can be happening at the same time the shot is kicking in so you may never actually see it in the #s, but hopefully that gives you some idea.
I have always thought of it as sort of a pressure variance, like a cork bobbing on the water. It takes a certain amount of pressure to keep the cork just under the water surface - if it is completely on top of the surface vs. just barely above it, the amount of distance it will travel is a lot different, but with the same cork & water it will take the same amount of pressure to keep it where you want it. That's probably bad science

and a bad explanation

but that's the image that helped me. Blue PSs have more upward pressure and the insulin is counteracting that rather than literally moving the #s down all that much. Of course that is ECID and depends on zoom rates & such whether or not that is true for a given cat.
The biggest thing I think to keep in mind is that if you are not seeing green nadirs or low blue ones, there isn't a compelling reason to lower your dose on PSs above your no-shoot. If you hit a nadir of 50 then yeah, lower that dose if the PS is a lot lower than you are used to, and collect data before testing higher doses. But if your nadir was only 250, you have so much room to go before you are in risky #s that IMO there's no need to lower the dose if the PS is above the no-shoot. (Barring cats where there is reason to believe the higher nadir is due to a dose too high.)