I have a new design I just got on my needles. This design will be approximately a square. When I cast-on for a square or full circle, I begin with a four stitch I-cord in smooth waste yarn. I make a few inches of I-cord so I have something to hold onto.
I then begin my design in the yarn I choose, starting with four stitches, directly from my I-cord. Once I have an inch or two of my design knit, I unravel the cotton waste yarn.
You can leave it just as you find it, but if the center hole is too large you can tighten it up.
And here you see the scale with my Chibi case and US 1 needles. All the supplies for this new design are here and I'll post some pictures of my move to a circular needle when I get there. Once I block this out, you will see a perfect center without any aggravation.