I've been confused by this information in the past. If someone said where to position the RR from the frame, (and I'm assuming that the distance of the RR from the frame rail is more important than the distance of the LR) and then they give you a rear tread width number, the LR will be where it's at and the distance to the frame rail would not be of any importance. If three numbers are given, it would be a rare instance when you could match all three.
From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)