Andy,
I am e-mailing Woolfman's vector calculator to you. All normals are calculated using 3 points in the order that they are coded.
Where you start in the sequence though, can make a difference. 98% of the time the first 3 normals work fine. Occasionaly it doesn't produce a good result, so I will start with a point farther in the sequence until the element displays OK - for instance in a 5 point element, I would start with the second and if that doesn't fix then, then I will start a calculation with the third point. You then take the other 2 points that are in sequence.
You are correct about the Y reversal, like I said in my above post.
Cheers