That's a huge fan! J/k since this the micro forum.
I usually just measure the outside diameter and then Google circle area calculater. Use diameter to get circumference. Then draw a line however long you want the thrust tube to be. Then draw another line for the circumference. So now you have what looks like a T. Then find out your FSA by measuring diameter of inner fan housing, convert to area. Then find diameter of the hub, convert to area and subtract the two. Now you have your FSA so times that by 85% and take that number and convert area to circumference. Now you have your dimensions for the end of the tube. Connect the lines. Make one side a bit larger by drawing a line parallel so you'll have a tab to tape. Also you need to arc the top and bottom lines upward a bit so when its rolled it keeps the edges parallel. The arc takes trial and error.
Or you could Google cone program.
http://www.conelayout.com/