The short answer serves two functions: i it provides hurried readers with an accessible, bottom-line prediction as well as the core of the relevant law and facts; and ii it provides the more thorough readers with an outline or digest of your subsequent discussion section.
Use whatever order makes the information clear and logical. The lawyer who gave you the memo assignment is busy. At the same time, bear in mind that the office memo should be a stand-alone document that can fully inform any colleague in your law office who may read it; therefore, the facts section should always contain a full and coherent recitation of the relevant facts, whether or not the principal reader of the memo already knows them unless, of course, you were instructed to do otherwise.
So do fifth. Since you are not advocating for any side, you ought not color or characterize the facts as you would if you were writing a brief.
What is the purpose of the memo? You may have also grappled with a seemingly contradictory assortment of facts: some seem to fit into the requirements of the rule; others suggest that the rule is not satisfied.
Writing allows you to record your short term memory into a format that you can examine and reflect upon, so you can suss out what makes sense, and how it makes sense, and then expand on the original seed.
Then give a brief usually no more than four or five sentences long self-contained explanation of the reasons for your conclusion, applying the rule to the facts of your case. Is the memo for general background knowledge of the law? This means writing the research in a way that's easily comprehended and digested.
In the rule proof you discuss cases to support the rule statement. Knowing this will affect your writing.