Well for the width-to-depth ratios, let's say you take a limb 10 inches deep. A 0.7 ratio would be 7 inches wide for every 10 inches of depth, so width = 7" and depth = 10".  If the image is taken from somewhere between a width cross-section, and a height cross-section, then the image's cross-section's length would be somewhere between width and depth.  The formula I gave assumes the cross-section's length goes steadily from width (0 degrees) to depth (90 degrees).  So if the rotation is estimated at 22.5 degrees (1/4 of 90 degrees) rotation, the length of the cross-section would be

7+(10-7)*(22.5/90)
=7+3*1/4
=7+0.75
=7.75"

So if you measure about 7.75 scale inches on the photo or art, finding width and depth would be like this

Ratio (r):
r=0.7

Intermediary between r and 1, corresponding to the image's cross-section (t):
t=r+(A/90)*(1-r)
=0.7+(22.5/90)*(1-0.7)
=0.7+0.25*0.3
=0.7+0.075
=0.775

Measurement (m):
m=7.75"

Depth (d):
d=m/t
=7.75/0.775
=10"

Width (w):
w=d*r
=10*0.7
=7"

a & b radii for ellipse:
a=w/2=7/2=3.5
b=d/2=10/2=5

pi Approximation:
pi=3.14

Approximate Circumference (C):
C=sqrt(2*(a^2+b^2))*pi*r
=sqrt(2*(5^2+3.5^2))*3.14*0.7
=sqrt(2*(25+12.25))*2.198
=sqrt(2*37.25)*2.198
=sqrt(77.25)*2.198
=8.789*2.198
=19.318"

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For the height ratios, it's just % of total height.  You either need to guess what your character's height is, or you can use the real height of the person you are morphing or otherwise basing the art off of.

Thanks very much for helping crunch the numbers, and show the terms.  While I don't draw this stuff, it's good to know when I put the physique into words. 

I may not have perfected the method (as one could imagine it is pretty hard to get exact off just a picture), but I have put together a spreadsheet to be used for approximating weight off a picture.  Far from an exact science, it at least beats trying to guess the weight just by eyeballing.  Right now it is hosted on my Yahoo! group (you have to sign up for the group).  If anyone wants to host it elsewhere, be my guest.

Weight Measurement Aides

Here is an illustration of the volume zones used.


Of course, part of the fun of drawing amazons is guessing the weight.  Even so, I grew tired of just guessing and wanted a better method.  My guess have been pretty accurate for the two fantasy girls I tested out.  I also tested it on a few real FBBs, and the weights seemed to hold up.  With the inexact method, there will still be some guesswork involved, but now you can at least have a "neighborhood" to place your guess in.

I love this thread.