I worked weight and balance at JAL in SFO for a while, the way we did our load plans was actually quite simple. Figure out how many pax are booked, estimate 165 lbs each, and two 30 lbs. bags each. Figure so many bags per LD3. It was all a guessing game for the first two hours or so, until we got our cargo final. Then we'd make some final adjustments to the load plan, give the thing to the dispatcher, and he'd take it from there. It was actually pretty cool to watch it all take shape throughout the process. Sometimes we'd be WAY off, sometimes spot on.
... and then we'd get the magical 26.0% MAC every now and them. That's when the dispatcher would usually take me to lunch after work.
... ahhh, I miss those days!