Don, with the greatest respect there is nothing wrong with 'rules of thumb' they have saved me once. Having said that, sometimes even then, the holes line up. In AKL with the SQ 747 (9V-SMT, since known as the "scrape my tail") both pilots were just off the A340. When the mistake of 220t for take-off instead of 320t was made it didn't seem strange to them. With all the distractions during pre-flight, fatigue, engineering, load control, Traffic, ATC and even the well meaning FA's with the very important 'drink order' you have to somehow slow it all down and double check everything. Anybody can make a mistake, it's managing mistakes that counts.
I like rules of thumb as a broad overview, a big picture thing.