Discussion section: Rough calculation to explain unexpected results
Generally yes. The purpose of a discussion section is to interpret your results, both in light of what was previously known (hopefully described in the introduction), and with new supporting arguments or hypotheses. A calculation (possibly short or rough) is a good way of providing a supporting argument, and it's quite common to see one in the discussion section of a scientific paper. However, theses are often subject to local rules, some of which may be silly, so make sure to check your institution's style or thesis guide, or check with your advisor whether they have any preferences.