Good article. I never saw any problem with how Marx reasoned through calculus as it was obviously an informal explanation. Uninitiated and malicious observers who have not otherwise read Marx will miss the dialectical approach he took in understanding the subject.

Qualitative analysis, critique of categories was always the larger concern for Marx. Given stable and correct categories, the quantitative answer is trivial to find (by this I mean not necessarily easy, but following directly from the question). But so shoddy was the theoretical basis in political economy that he had to theorize extensively to correct it.