Big sheet of paper, calculator, pencil etc etc.
After thinking about the idea that Steve had I realised that the angle of the cone remains constant so I could work out "X" from the distance between the 2 ends and the difference in their radius. Simple, after that.
After all the farting about I came to a solution that shows a certain simplicity to the design. The cone is 33 inches long, has 1/3 of it lopped off of the top and then 22.5 degrees of the resultant 33 inch radius cone is used (1/2 of a 45 degree section). The scaling of the photo was probably a little bit off but these new dimensions look close whilst having some 'order and symmetry' to them.
So, draw a 33 inch radius circle with an 11 inch circle inside, slice 22.5 degrees from it (1/16th) and add tabs to it as necessary.
Transferred the shape to a roll of wallpaper as a template for the cutting of the sheet. Now comes the fun part of rolling it into a cone!
Thanks, guys.
Rob