Traditional Sweaters (like the ones in Knitting in the Old Way), designs by the late great Elizabeth Zimmermann and her heirs in spirit are based on the measurements of the intended wearer. Not, thank goodness, on the "one size fits some and no size fits me" of the fashion industry.
So with swatch in hand and knowing that this sweater needs to be 54 inches around to fit me (including ease), I calculate that I will need 4 x 54 = 216 stitches for the body. The cardigan borders will be 10 stitches on each side.
10 border + 54 left front + 109 back + 54 right front + 10 border = 237
The math works out for 108 stitches in the back section EXCEPT that I need an odd number to balance the planned K1, P1 ribbing, so I put the extra stitch in the least obvious area.
Now if this was a pullover or if I wanted a cardigan that pulled in strongly at the waist, I'd start out with 10% fewer stitches, but remember the design specifications - loose fit to wear over other clothes. So I'm going to cast on the full complement of stitches. And I'm only going down one needle size for the bottom ribbing, not two sizes which is the more 'standard' for ribbing.
Pardon me while I go cast on........