Our Add-In requires that we translate coordinates on a Map to coordinates on a Layout that contains a given MapFrame. Per a previous discussion, this has to be done manually, at least for now - unless there is some way I'm unaware of to easily ask the SDK, "Where is this MapPoint represented on the page within this given map frame?"
That in mind, I'm using the map frame camera's Scale property, along with the camera's spatial reference's unit and the page's unit, to determine a number that represents the "page units per map unit." The linked post includes an example that assumes map units in feet and page units in inches, but this assumption won't necessarily hold true. I use the "page units per map unit" figure to relate real-world map distances to distances on the page.
I'm not sure how to apply this scheme (if it can be applied) to situations where the map frame's map and camera have angular units. What does the camera's Scale property really mean when the camera's units are angular? API documentation describes the Scale property as "a ratio of distance on the map to reality." Is there some way in which, for instance, decimal degrees are being "converted" to meters?