I just did the same, twice; and I agree. A very reasonable alternate method.
1.) I decided the center of the car was halfway between the inner wheelsets.
2.) I used the length of 40', as in the ORER.
And I ran my calcs again (twice), and got the same dimension (6 3/4'). In this case: (20 x 3.3) / 9.5 = 6.9
Ah, well. 'Tis a puzzlement.