Apart from the heavy jets where the burn will vary dramatically as weight reduces and altitude increases the effect will not be large between Max AUW and MLW. 12 tonnes in the first hour and 10 tonnes an hour and then 8 tonnes in the last hour is just what I need, or an average of 10.5 tonnes per hr.
All I'm after are some "ballpark" figures: the 738 burn will vary between 1950Kgs per hr to 2500 kgs per hr. However, the average ( average weight / average day / average cruise level / etc) is about 2100 kgs per hour ( it burns more in hour 1 than hr 2 due to climb and less in hr X due to descent).
If you were a new F/O and asked a TC "What does it burn an hour?" you would get an average answer; that's all I need for the time being.