I also think you have a misunderstanding. Adam Smith is clearly explaining the reason for the more liberal wages from the perspective of the one who is to commence the study, and his likelihood of succeeding. Smith is pointing to the fact that liberal professions are inherently more difficult to master, and often provide a value that is more rare (and therefore worth more money).

You cannot turn this quote into a formula, because it's a description of how wages ought to be, and the reasons therefore. And it is completely irrelevant to discussion, as I'm sure if Smith were alive today, he would count the art of programming among the liberal professions. There is more to making a useful application than to be able to think mechanically. Because programmers create and shoemakers copy.