Your entire post is pretty much money! There are no set protocols to any aspect of bodybuilding, or anything related to genetics. It would be virtually impossible to come up with an accurate reading on someone's genetic potential. Even if a geneticist did a DNA test on a person and used a punnet square to find out what that person's genotype capabilities are; all that geneticist could do is guess or predict what the phenotype "could possibly" express.
Moreover, that geneticist would need the input from expert: nutritionists, chemists, and mathematicians to calculate all the variables. Furthermore, with all the data calculated from the experts from different fields combined; I would venture to say that it would be hard for all of them together to come up with a "rough estimation" to what would likely occur. All they could do is "predict" the outcome, which is not concrete evidence. Their summation would be abstract, hypothetical, and conjectural because there are too many uncertainties and not enough certainties to give a unequivocal answer.