I'm using the sheet metal function to create a rolled bar so I can have a flat pattern.  However, the flat length of the part is not correct.  SW's is giving me the flat length of the outside radius arc length, not the neutral axis.  What am I doing wrong?

Need to roll a 2x4 steel bar in the hard direction, i.e. 4" direction with an 88" outside radius and an angle of 90°.  This will give a flat length of:

Rad of neutral axis*2*π*(angle/360) = 86*2*π*(90/360) = 135.09.

SW's flat length derived from the outside rad:

88*2*π*(90/360) = 138.23

I created my model from a sketch with a 90° arc and 88" rad.  Then I used the base flange feature with a direction 1 of 2" and sheet metal thickness of 4" and a K-factor of 1 to ensure SW doesn't add a bend deduction.  SW automatically creates the flat pattern with the length equal to the arc length of the outside radius.  What am I doing wrong?