I'm having trouble working out what the '+ b' part should be in the formula X=Y m + b, where X is the arm to be found, Y the calculated weight, m the slope & b a constant: Is it the distance up the Y axis where the line would intercept at x=0?
If X=Ym+b, then b=X-Ym.
If you know X and Y for any point, and m (the slope) then it's easy. If you don't know m, then you need two points - we'll call them 1 and 2, so..
(#1) X1=Y1m + b
(#2) X2=Y2m + b
Subtract #2 from #1, and you get...
(#3) X1-X2 = Y1m - Y2m = (Y1-Y2)m
Divide through by (Y1-Y2) and you get...
(#4) m = (X1-X2) / (Y1-Y2)
Plug this back into your original equation and you get...
b = X1 - (Y1*(X1-X2)/(Y1-Y2))
G