I read a lot of the posts and have to agree the the best way to determine the thinning of the paint and the rolling technique is to try it yourself. It is sort of something you get a feel for. My advice is to just go ahead and try it- better to thin than too thick...
Things I learned:
1) If it doesn't run on the vertical surfaces it is too thick
2) Better to apply it to thin than too thick
3) If it runs, then that's good and go over it again in a few minutes with a roller with less paint to pick up all the runs
4) When done with the paint session, go over the entire car again with the nearly dry roller to take out any runs- especially the places where the runs accumulate such as the door creases on the vertical surfaces
5) If you get any runs, then it is much easier to roll them out while the paint is wet than the sand them off when the paint dries
6) consistency of nonfat milk worked for me
7) best bet is to have thin paint, but very little on the roller (roll it hard on the tray 6-8 times until no more paint squeezes off) before applying to the car
8) use a new roller and foam brush for the edges with each
application
9) It takes me 60-75 minutes to roll my car each time, and I have to add a bit of mineral spirits to my tray every 15 minutes since it evaporates and gets to thick

Hope this helps!
My White Comet so far:

Marc's White Comet Project