CCD Stack - Missing Values

I purchased a larger apo refractor recently and decided to attempt another composite M31 image using missing values construct per Adam's tutorial.  I shot 35 3 minute subs at 180" and a like number of subs at 15".  I processed each stack using CCD Stk and then refined both images using Photoshop extended plus a few plugs ins.  I normalized both images with the 180" image on top.  The resulting weights approximated the exposure ratio.  I used 40,000 to 100,000 to create the missing values range.  I also did a 5 pixel grow but whne I finally did the combine operation, the nucleus area was distinctly brighter than the abutting area at 180".  Should I have not  processed the two images through PS CS6 extended before running the missing values construct?  I tried  to up the minimum value restrict range to 45000 and 55,000 but that failed to remedy the mismatch.  Any thoughts would be welcome.

Frank P.