My guide in Maine had a chart based on the circumference of the chest. Maybe you can google it or something. It was pretty accurate. we killed a 518lb black bear and the chart put it at 530lb, if I remember correctly. So, it was pretty much right on.
Way too many variables to guess weight by measurement.
In the field, judgement (besides the apparant) of size is typically based on spacing between the ears.
Estimating bear weight
