I don't understand your comment. "assumptions used by peoples"?
Does anyone REALLY believe that study? Or do you just WANT to believe it?
Have you read it? What is not to believe? You don't think the millions of illegals here today will bring some family? You don't think the increases in legal immigration limits will result in more immigration? What do you find unbelievable? What assumptions do you find unrealistic?
Cranking up a model assuming certain parameters and letting the computer run iterations and extrapolations can be fun. However, let us not put uncritical faith in such things.
The Heritage paper should get the same critical evaluation you would give one which came to the opposite conclusion. If you do this I do not believe you will honestly accept the claim that another 100 millions will come here. Not even in 20 years.