I dont see why this couldnt be done in. Just need to define what is “close” and then when to use the midpoint. Then is it the midpoint of all available bodyweight cals for the MRN, or is the last 2, 3, etc… Need to be specific here so DAX “knows” what path to take.
Also, some sample data in a PBIX file would be preferred. Along with a quick mockup of what you would want/expect the out come to be.