knitting a pillow cover for a 12x12 pillow

[bethie]

Hi y'all. I'm fixing to make a pillow cover for a pillow form I bought. The pillow form is 12x12. It is a bit puffy.

I figured I'd knit a front piece with a pattern and just a plain back. On the back I'll probably make some button loops so I the "pillow case" will fit closely around the pillow. Since the pillow is a little poofy I thought this would look nicer than an open ended pillow case.

Since the pillow form is not perfectly flat (I picked it up at Michael's Arts n Crafts), should I make the front and backs like 12.5 x 12.5?

On a side note, I am proud that I understand gauge (stitches & rows per inch) and will be able to alter the borders of this pattern to make it be 12x12 or 12.5 x 12.5 I already did the math in advance with my trusty calculator.
http://www.knittingknonsense.com/b_cloth.html

I really appreciate the friendliness and guidance I have gotten so far. Y'all are the best!

Bethie

[Ingrid]

I think I'd measure from edge to edge around the curve of the poofy part of the pillow to see if 12 1/2 will be enough. Make sure you make an allowance for the seams, too, though.