Excellent question and one that we have some research data on. You are looking to substitute forage from pasture with distillers. Nebraska data would suggest that, on a dry matter basis, 1 pound of distillers will replace between .4 to .6 pounds of forage. Because distillers is about twice the energy value of forage, we would have expected that 1 pound of distillers would replace close to 2 pounds of forage on a dry matter basis. In this case, I think vegetative forage and distillers are feeds that are high quality and therefore highly digestible and do not have to reside in the rumen very long. So rate of passage is also high, and there is not much of a "fill" effect from feeding the distillers.
If you are looking at using this management technique on yearling calves on grass, the price is right for distillers, and you know your fuel and labor costs to deliver the distillers, this forages replacement is not a concern because ADG will be increased. So in this situation, more total pounds will be gained by calves on pasture. The increase in ADG depends on the amount of distillers fed. You need to pencil out the input and output. Go to this beef report to look at performance of calves fed distillers and a high quality hay to simulate a forage grazing situation
http://beef.unl.edu/beefreports/200506.shtml(PDF).
It appears that for each pound of distillers fed on a dry matter when fed with a high quality forage, ADG inceased by about 0.15 pounds. The line is linear for the amounts of distillers fed in this experiment. There is a point that it is not linear.
