How do you measure for coil overs?

[Over2tonsoffun]

I am looking to do coil overs on my new project. Viking has them for the front but not the rear. So I figure to use a set of universals ( if there is such a thing) for the rear. Only problem is how do I know which length to get. What measurement do I use, The factory coil, the shock, other????

Thanks
Tim