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.
What kind of light source do you have on the enlarger?
I do use the ISO triangle to find my speed. Then I stick with that speed.