ASHRAE Modified Zone Method

I've been trying to determine U-factors for residential building components using the modified zone method described in the ASHRAE Fundamentals Handbook.

I've written a spreadsheet that works well, and can duplicate results published by Oak Ridge National Laboratories, so I think I'm on the right track.

My questions are: Is there a limit to the sheathing thickness that can be considered using the modified zone method? For example, if I have 6" thick foam sheathing, is it correct to only use 1" in the modified zone calculation and then add the other 5" layer in series? What about for cavities deeper than the 6" shown in the zf chart?

Also, if the modified zone method is only for clear wall sections, how can I apply an appropriate correction factor to account for the additional framing for a whole wall U-factor?

Thanks for any replies.