I recently made pillow cases such as you describe. I followed a tutorial on you tube (
http://www.youtube.com/watch?v=FhZnHhbcN-Q). I know you said you didn't have sound on your computer - so I listened to the measurements part to refresh my memory.
It was recommended that you cut the fabric for the front 1 inch bigger on all sides as is the size of your pillow (so for a 25 x 25 inch pillow - 26 inches). For the back, the same 1 inch plus add 4 inches (this is for the overlapping part - the envelope). So for the same pillow, the fabric for the back would be cut 26 x 30. You cut the back fabric in half.
If you have a really full pillow, I am thinking an extra inch or too can be added. Which I am really saying about the same thing as puddingtain.
I was using leftover quilt blocks for my pillow cases - so my front/back fabrics ended up being a little bigger than what was recommended in this tutorial. By a couple of inches I would say. But the cases turned out nice all the same.
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