Well back when they were popular they were usually called "Slapper Bars".
There were articles in magazines like Car Craft, Hot Rod and Popular Hotrodding on the subject at teh time.
I would think that starting out with somewhere around a half inch with the car sitting static on the ground and going from there. Then adjust to personal choice as you test it out.
There were also bolt on spring clamps = a pair of flat bars with bolts through the ends that were popular in the 60's and 70's but I don't think I would want to run them on the street.
The hot Mopars usually came with an extra leaf in the right spring as a traction aid also. As mentioned above Mopar did a lot of work to make their cars hook up right out of the box.