Soft Question: Documentation of Important but Commonplace Calculations
While my academic background is in theoretical CS rather than "pure" Math - it was mostly combinatorics and probability, so I feel qualified to tell you that:

AFAICT there isn't a standard. I suggest:


Leave calculations private? Definitely not!
Write a blog post - Most people don't have a blog. Also, a blog is not the most accessible medium for results in Math, as online resources go. It won't show up in places like, say, Google Scholar.  Also, some blogging platforms are not very stable over many years. Bottom line: If you do blog about things like this, possibly write a blog post in conjuction with something else.
Write it up on Wikipedia If the result itself is basic and commonly-used enough to merit a Wikipedia page, then sure. But - it might only merit a mention in some other page, in which case it doesn't make sense to try and stick a proof/calculation in there.
Write a so-called "note" on something like arXiv  ---> Do this &lt;----. Collect severall related mini-results / calculations, describe them nicely, introduce them with a bit of motivation perhaps, find a reasonable title and publish it on arXiv. 


One last option:


Sneak it into some book : This option is not available to grad students, but if you're a more experienced academic writing a book (perhaps something introductory, but not just in that case) - put this stuff in an appendix. A great example is Appendix A of Alon and Spencer's The Probabilistic Method, about large deviation bounds. The book uses those all over, but isn't about these strictly-probability-theory results, so they're not a proper chapter.