A portal does not send people between two places, as a Teleport or a Gate does. A portal reshapes space to make two circles the same place.

A portal does not have two parts, or even one part that is in two places. There is no longer any fact of the matter about the former two places being distinct. Any effect that relies on the distance between two points will work through a portal as if through normal space, because it is.

A portal can be dispelled by targeting the area containing it. It cannot, itself, be targeted, because it is not an object, or even a thing. It is just - an area of space, which happens to have a complicated spatial metric around it.

To make an interplanar portal, Ahmose found the part of the spellform which targeted the effect, and - disabled it. At first this spell did nothing, but with enough prodding he made the spellform say, in effect, target any place which cannot be specified by the normal mechanism. Why this made an interplanar portal, and not e.g. one leading outside the one-mile range, is not something Ahmose can say; but he knows, now, that it did so.

He doesn't know how to make it target the place he's thinking of, instead of a place he's looking at. (He tried closing his eyes and thinking various thoughts; the spell failed.) But he can change the targeting component in every way that still stabilizes, and - see what happens.

Most changes to the targeting component fail harmlessly, or try to fail not so harmlessly and are caught by the wards. But some are - usable.

One change makes the two sides of the portal non-parallel. Not useful.

Another change identifies a circle of space with itself. This does nothing, as far as Ahmose can see, but the spell is still hanging there. Not useful, and he should be focusing on changes to the interplanar version anyway.

A change which... doesn't seem to do anything? The spell still opens an interplanar portal, to the same destinations as before and seemingly at random. It's a different spellform but Ahmose can't see what changed in the outcome.

A change that makes an interplanar portal go the same place as the last one he cast, without him having to specify where that was.

...success? No, because the portal back home that he wants to recreate was many portals ago.

AAAAAH, he doesn't think because he's very focused, and goes on methodically trying every change he can stabilize in what he hopes is an exhaustive ordering of the possible spellform variations.

Time flies quickly when you are incapable of focusing on the passage of time, or caring.

An hour and a half later, Ahmose abruptly notices his ankle hurting because he had been standing in an uncomfortable position. He was aware of this before, but didn't move, because it wasn't relevant to what he was focusing on.

Objectively, two new modifications are an excellent result for ninety minutes of focused study. Subjectively, Ahmose feels like he might have strained something. In his head, not just his ankle. The spellform is more present, more available in his thoughts now, even when he's thinking of something else. It's like an image of the sun that burned itself into his eyes after looking unblinking into the light. (Hopefully it will fade like one, too?)

He dully duly reports his findings to Jasper.

I think it might be a good idea for you to rest now. If you were a Magical Girl, I would say that was an extremely productive training session.

He probably should, shouldn't he. He certainly can't go on experimenting while he vividly remembers being like that and - not being like that, anymore. (Can he ever again?) He'd be far too distracted, not just by the usual things but by the fact he's distractible.

The local time seems to be early evening. It might be later than that for him, he hasn't been trying to keep track of time and it feels like days since he woke up, with everything that's happened.

"Yeah, I - should probably go sleep. And rest." 

(Another nice thing about the not being distracted was not worrying whether he was spending his time doing the right thing.)

As soon as you put the Chicago portal back, you can sleep. But it's alright to take a few minutes to recover first.