How closely can I follow a cited mathematical derivation in my PhD dissertation?
The appropriate wording for something like what you envision -- making clear that you are closely paraphrasing another publication -- would look like this:


  Since the derivation of the result by [ABC, 1956] is not widely available in common textbooks, the remainder of this section closely follows the excellent outline provided previously in the MSc thesis by DEF, see [DEF, 1987]. In this thesis, the author shows that ...