I would prewash the flannel. Based on my experience -- all flannel shrinks and most flannels shrink at different rates (even within the same brand) I agree with buying an extra half yard (at least). I have had flannel that started out 42" wide go down to 37" after being washed -- and don't get me started about how the length shrinks. I have had shop owners claim their high end flannel does not shrink and find it hard to believe when I show them proof.