So far as I know, the mapping of these real-world units of measure to “BU’s = Blender Units” is arbitrary. I fully expect that the multipliers for “feet” vs. “meters” are correct.
The extremely-important lesson is simply: “pay attention to scale!” Grab that 100’ tape measure and actually measure things, and make sure that everything is “to scale” in relation to everything else.
When I’m roughing-out a scene, initially the objects are just simple geometric shapes. But, they are the right size. And, they are placed on a set which is also “the right size.” “This room is 11 x 14 feet with an eight-foot ceiling. That window is 2 x 4 feet and is 36 inches above the floor.” You really can’t just “eyeball it” and have it come out right.