Would I be correct to make a guess that most flights take off with similar weights? I mean, if I was running flights across the country, I would look at how many pax there are, then add in roughly how much fuel they are planning on taking, and then fill the remaining available weight with freight.
It varies a fair bit. About the only flights that have consistent weights are those like the 94, which tend to be at max take off weight every day. Fuel loadings are available as averages of the past, but actual the actual order isn't made until about an hour before departure, when the Captain sees the flight plan. Boxing him in, by filling all of the available weight with cargo (and so allowing no extra fuel above the minimum) could certainly be done...except that he could then simply offload whatever weight of cargo he felt like removing to make way for fuel...the fuel order is legally his, not an anonymous planner. ...