So you don't really know your AUM when you are flying then?
In the real world flying some do everyday.....that is a very common occurrence.
A few examples....
You land on the Oil Rig and are given a Manifest showing numbers of passengers (using standard weights) and a Baggage weight (presumably weighed).
There comes Bubba dragging a Duffle Bag and carrying a suitcase and Tool Box.....in company with his brothers....all of whom look like the Incredible Hulk's. big brother.
You reckon they are "standard" weight....and all their kit weighs 20 Kilos each?
You land to pick up core samples of rock at a hilltop core drill.
How do you weigh that bit of cargo?
It is easy in a well structured environment to weigh passengers, cargo, baggage, determine the Temperature, Elevation, Wind, and distance to and height of obstacles......but not in the real world of utility flying.
You can weigh the aircraft by using the weight and balance information in the RFM and Aircraft Records, check the elevation and OAT, then reference the WAT Charts but after that it gets a bit ambiguous as to what you are dealing with sometimes.