I wasn't aware you could ever get negative AIP, but I don't think that's what you mean.
And as far as the numbers are concerned, it's doesn't matter in which order you get data centers, co-procs or terminals. Your AIP will always be your (total AIP - AIP reduction) but will never drop below the floor. If you have 100 total AIP, your floor is 20. If you pop 4 data centers, your working AIP will be 20 (100 - 80.) If you pop a 5th dc, your working AIP will not drop below 20 because that is the floor. If you then take a planet, however, your working AIP will still be 20 (120-100.)
Now, start over. 100 AIP and you start milking a terminal. Again, we'll ride it for an 80 AIP reduction. What actually happens, though, is that your total AIP goes up 80 while you get a reduction of 160. Because your total AIP went up, the floor went up as well. So, your total AIP is now 180; your floor is now 36 and your reduction is 160. Your working AIP ends up being 36 instead of 20. Again, take a planet and your working AIP will be 40 (200-160.)
Co-incidentally, co-processors increase your AIP in the same fashion and both are very inefficient when compared to data centers. I've always felt that it's a bad idea to go with the least efficient option in the early game but really, you're going to use every tool available to you in the game to reduce your AIP so the only real question is; how long are you going to leave it on for?
I hope I helped answer your question, I couldn't quite understand what it was.