how much lift for 40 inch tires?

[ProGrade]

put the tires on it, and then start turning the steering wheel, if nothing rubs, compress the suspension and try again.  

whenever you do rub, measure how much travel you have in the area you are rubbing, and you will know how far to lift.  if you have less than 1-2 inches of clearance during this check, consider it a rub spot.  parts will move more when they are under load.

I used this method when I cut my fenders, and I never rubbed where I cut until my body mount fell apart.  I run 35' tires with no lift.