How do you calculate a/c sqft?

First off, what good is square footage? For HVAC calcs you should be more concerned with cubic footage. Secondly, whether you use the interior or exterior dims or the wall thickness isn't really going to make that much of a difference in the overall requirements, which is what you're probably concerned about. Do the math both ways (using interior then exterior) and you'll see that the units required for both will be close if not the same.

If you are sizing the unit requirements, just take the overall exterior sq. footage of the space to be condition and multiply that by the height. Then if you are sizing individual rooms for ducts and register sizes, use the interior dims. Naturally, these totals won't be the same, but you're overall cubic footage should be larger than the totals of the cubic footage for individual rooms.

A/C load calcs are laid out in ACCA manual J for Residential and ACCA manual N for commercial. In a nutshell the exterior dimensions are correct. The thickness of the wall and materials used will impact the heat gain/loss in the structure.

While not a draftsman I have drawn (old fashioned way, no CAD here) several homes and when calculating load for heat & Air demand you have to consider cubic feet rather than square feet. Years ago (60's) you could mostly assume 8 ft height for walls whereas for the most part today people are using other heights or varying them from room to room not to mention many rooms have cathedral ceilings. This extra height should be considered and is far more important than the 2 to 4 inch difference there is in exterior walls. I can't imagine that being much of a difference unless you are calculating a 5000 sq ft or greater home. For instance say a 70x70 home with 4 in walls compared to a 70x70ft home with 6" walls your difference in total sq ft would be 23.47 sq ft whereas if you factor in cubic ft given height of interior walls (assuming uniform height throughout the home, for example only) the same 70x70 ft home with 8 foot ceilings compared to 70x70 home with 10 ft ceilings the difference is 9800 sq ft or 363 cubic yds. Also when you calculate for H/A loads I believe you also factor in R-Value of the walls depending on what the construction is. If SIPs then the R-Value is pretty constant throughout whereas regular insulation can be reduced to an R4 depending on installation and compromises.

NO expert here but going in part on what a friend shared with me when I went to draw and build my home.