My understanding is that the U.S. Forest service green log weight chart, which forms the basis for what we use, is based on averages, from across the country and different calculation methods.
As such it would be representative, a cross section if you will. (Pun fully intended) Of course I realize the problem with averages. Half the people I know are below average!
I have always found it useful when estimating foliage to take the lenght of the branch and calculate based on its greatest diameter. For instance a 40' limb that is 20" in diameter at its greatest would be calculated as a log 40'x20". I am usually within a 200lb. No scientific, not fullproof, but a start.
Tony