Procedure is like this:

Passengers are assigned seats on their tickets so that atr is within w/b limitations. We get an electronic w/b closeout over ACARS that lists how many people should be sitting in each section. Separately, the FA's do a headcount themselves and call us with the totals of each section. I then compare the FA's numbers to the electronic w/b and make sure everything is kosher.

Problems normally arise when people don't sit in their assigned seats, etc. Also, it would not be easy or feasible to teach FAs where to sit people on partial loads. We have four different baggage compartments (five if you count C4A and C4B as two) and they are at the extreme ends of the aircraft. How would the FA's know where the bags were/how full they were so that they could seat the passengers accordingly? Furthermore, this would be a cluster F with 50+ people on board....anyway you get the idea. Not always as simple as it seems.