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http://ecomodder.com/forum/showthrea...tml#post263738 has an updated procedure that I used( including math and measurements on how I did it. Actual measurements should be scalable to whatever your height is.
Basically, measure the height of the vehicle, figure out the 10% locations, start building the tail to get close to the height/angle at that percentage, adjust final fit once the structure is in place. I have a dial angle finder that I use to check against the floor of the shop.
Hope this helps.