If the government of smartypants wanted to subsidize college education, how much would it subsidize each student to achieve the socially optimal level of education?

I think it would depend upon the financial background of a student. A good family financial support does not need a big amount for education. The US offers part time job opportunities that are favorable for them. The percentage should still look into a financial assessment for school education and have a definite clustered support that is based on their capacity to go through the whole course.