Bill,
thank you for the explanation. It is much more clear now.


Christoph,
I think he refers to 21 steps stouffer because OP says that he uses 21 step http://www.apug.org/forums/forum44/8...-exposure.html

mine 21 tablet step 11 = 1.54
31 tablet step 16 = 1.57

either way step number 11 on a 21 stouffer should be the same as step number 16 on 31 stouffer
I think you are right, they should be close