Okay, you can't get proper gauge from just the Cast on as stitches are tighter there than they will be when you knit them, they would measure about 80% of what the finishes stitches will. And one row won't be accurate either, you need at least a couple of inches.
So here's what you do since you're going to redo it anyway --
CO about 24 sts to just one needle, knit across it, then slide the stitches back to the beginning of it (which you probably won't need to as they'll pretty well fill it up). Then leaving the yarn from the end of the row loose across the back (very loose) knit another row. Repeat for about 3". This method mimics knitting in the round but lies flat so you can get a good measurement on it. The stitches at the edges are going to be very very loose, so don't use them at all, just measure 2 or 3 inches across in the middle. That should be enough to tell you what your gauge is.
sue- knitting heretic