According to my non-professional reading of Jim Griffin's white paper, the center to center separation of circular drivers yields to ACTIVE RADIATING FACTOR(ARF) AND SLOT (RECTANGULAR) DRIVERS. The flange detracts from the active area in the array, and any space between the drivers. So the ARF is the total percentage of the active area in the array from the top of the tweeters to the bottom of the tweeter line. If you used a single long thin driver horn, then the ARF would be 100%. Stacking them takes away ARF %age, but cutting flanges increases it. According to the values in the white paper, the ARF needs to be 80% or better.
RIBBON PLANAR DRIVERS use another method.
eric j.