Cutting up a rear axle.

hey cracked. i am just looking at the spread sheet you emailed me. i am a little confused the total axle lengths widths you have down. how come you have the dana's total length 9.45" shorter than the 8 3/4? Have you done this for a certain tire/wheel combination? I am still unclear.

Should a guy start at the tire/wheel combo of choice and work backwards?