Yeah, you should have more stitches so the center part you measure is nice and flat. Since you're measuring over two inches (many patterns measure over 4) and the gauge is 15 over 2 inches I'd probably cast on 25. Then knit a good 2-3 inches.
Also... I always do garter stitch on the edges and top and bottom to make it lay flat. In this case I'd probably garter 3 or 4 rows on the bottom and 3 stitches on each edge. This is all assuming your gauge is done in stockinette and not garter.