I run into this a lot being I build a lot of one off stuff that creating a fixture or jig for is impractical. I compensate with strong backs clamped to the work piece. This can consist of a couple pieces of angle iron doubled up and make shift trusses using other rigid extrusions and shims to add compensatory stresses in certain areas. 11 gauge is easy to correct. You may find that you can set the top on the floor upside down and while standing on it, move the leg wherever you want just by pushing on it or whacking it sharply with something with some heft to it. If it's the top frame you need to tweak, you could clamp some angle on the side you want to keep as is, and jack against the side you want to move with something as simple as a bottle jack from an automobile tire kit, or even driving a 2x4 that is slightly larger than the finished size or something.
Free building things without jigs certainly has challenges, but typically, if something starts out in the right place it can be put back reasonably. Every time I get a drop of some really straight 1/4 wall 2"x2" angle, I-beam, or anything like that, I stash if for makeshift jig parts and strong backs. That, along with an ample collection of R-11's and scraps of 1/4" flat bar become invaluable.
In place of a middle leg, if you really need the extra support, don't discount a truss instead. It doesn't have to be bent tubes like this. Even a girder between legs and some nipples between it and the top will add a considerable amount of strength.
On this 12' table, they opted for a middle leg on the rear to save cost, and a truss on the front. I had to build pre-stress into it because it's only 1-1/4" sched 40 aluminum and will have some weight on it. Adding the leg in the middle and fully welding it would kink the top so it had to be backed up with a strong back while welding, as did all the oval nipples on the front truss or it would have looked like a stick of bamboo if left to fly by itself during the welding, or even tacking stage. As it was, I had to still do a considerable amount of correcting both during and after the fact. The fab table in this situation wasn't really much of a help other than to hold it up higher to weld it easier.
If you notice in the photo, some of the legs are still crooked. They didn't start out that way. I can't scratch the metal either, so it takes creative tweaking and it would, even if it had been built in a fixture.
You may consider adding adjustable feet to your table legs any way. I have rarely found a floor or table that match. The fab tables in the above photos have them. That concrete floor there is awful. 1/2" difference in 8 feet in some places.
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