When an engineer working on a project I was managing left the company and we looked for information in his files, the only design calculations recognizable as such were those produced by various vendors' equipment sizing software programs. We needed design data to respond to a question from the client regarding system sizing and it was embarrassing, to say the least, that we could not immediately provide an answer.
In the "good old days" when I was an engineering student, professors still taught that a complete and clearly written problem solution was as important as getting the correct answer. Back then, professional engineering examinations consisted entirely of written problem solutions, and this rule was emphasized as critical to obtaining a passing score.
I still find that writing up a formal calculation to document a "back-of-the-envelope" design computation often provides an opportunity to realize that I've overlooked an important factor, or made an unthinking assumption along the way. This benefit is obtained from a calculation structure that forces a clear statement of the problem, and the assumptions and methodology by which a solution is to be reached. While specific format is a matter of personal or organizational preference, a good calculation should address the following key elements.