The gauge swatch is 21sts x 28 rows for 4inches in cell stitch but the cast on is 69sts for 26.75 inches (10.3sts/4")? Does the stitch number increase after the cast on? You would think that the cast on should be more like 140sts if it's all or mostly all in the cell stitch pattern.
Even if you're right on gauge, I wouldn't take the chance for only a slight gain in width and length. One extra ball of yarn is a safe buy for an afghan anyway. If you get to 35.5", you could always lengthen the afghan a bit with the extra yarn and you might end up needing it anyway..