The formula is apparently to make the slot area around 1/3 of the cone area.
Thanks
Assuming I did it right I come up with:
Surface Area of Cone (Sd) of Eminence 15" Definimax is: 872.9 cm²
Slot area = 1/3 cone area is: 291 cm²
Conversion to inches is: 45 & 7/64 in²
Guessumed slot width is 16"
Indicates a 2.82” or 2 & 13/16" internal slot height
Appears to jive with the pictures that have been posted.