Solidaxle, I am by no means an authority on linked suspension. But my understanding is that the instant center is the thoretical line where your links would intersect if they were continued along there current axis. I've been told that shortly behind the tailshaft (on a hot rod) is the "sweet spot" to shoot for, but it needs to be within 50-80 percent of the center of gravity (distance from the middle of the bell housing to the ground at ride height) in order to keep your anti-squat from being too great, or too little... I ran the mason's line along my intended axis to be sure that I kept my upper links parralel to it during installation.
I know, more mud, sorry. Not very good at explaining. Here is a link to a 4x4 site with much better information on 4 links
http://www.therangerstation.com/Maga....htm#AntiSquat
Once you've read that, and understand it, here's a link to a fancy 4 link calculator on Pirate...
http://mysite.verizon.net/triaged/4l...tml/index.html
Hope that helps