Most Critical M & N Couple for SLS Analyses

Hi!
My problem is, in my opinion, should be very common, but have not seen many posts.
I have a shell and with most software, I can get an envelope of results along many load combinations. The structure I am working on is mostly governed by SLS (Crack Width). So, I go in and get the shell results envelope and get the minimum axial force with its corresponding bending moment and maximum bending moment with its corresponding axial force. However, these MN couples do not govern the SLS analyses, values in-between these two edges govern. I have shown this in the following figure. The orange line is capacity for given crack width criteria. As you can see edges do not govern.
So, I am trying to find a way to simplify the selection of "design couples". I have tried M/W+N/A, it also doesn't work. Does everybody just go and get the maximum and minimum? (Please don't comment on the "reliability of crack width calculations is very low so why bother" etc. I am trying to find the correct way to reach most critical value.)