Well there is a lot of opinions on how to do it. LOL
All I know is how JD (Sunflower too) measures theres and how the sales manual stated the widths when you ordered one. YOU DO NOT measure the back gang width!!!! It is the amount the front cuts PLUS one disk spacing. The easy way is to count the blades on the front gangs an add one then multiply that times the space between the disk blades.
Example: Lets say you have 22 blades. So you would take 22 + 1 = 23. The disk blades are 7 inch spacing. 23 x 7 = 161/12 = 13 foot 5 inch cutting width. The back of this disk would measure almost 16 feet with furrow fillers. It would NOT be called a 16 foot disk.
You are measuring the amount of NEW ground the disk will cut not the amount it levels.
If you don't believe me then look up the cutting width on JD or Sunflowers web site. Then look at how they figure it. It will be like my example.