If you are running a set of 700X40 tires, you can generally run a 650X48. ..
Not true. I don't want to mislead people. I can run max 700x40, or 650x40. I just don't have any flair in my chain says (carbon). Some of my bikes (crimped aluminum), have less clearance with 650, because it moves the wheel behind the crimped section
Obviously the UP and 3T are designed for 650b, but many/most bikes are not - it depends on how the chain stay flairs (and to a lesser degree component interference and seat tube clearance (and in some cases seat stays and brake bridge)).
I'm guessing your rule probably works on the front for, but not for the rear clearance.