OK Bill, of course I have no calibrated sensitometer. I test under my enlarger. You know like it is explained by BTZS. I hope some thing are becoming clear now. BTZS uses the ISO triangle and I always wondered why I was getting more speed than by using the ZS method. Also having problems with flare. Which Stephen explained nicely.
I think I am slowely starting to see the light. Dimly but it is there.
Concerning the Delta-X criterion. Wouldn't it be better to use the full length of the curve (Zone VIII) and mathematically calculate the values we would have at using the ISO triangle.