Check post “28” on the same page. There is a fairly good image posted of what is happening and what you are trying to accomplish by “stretching” the frame out.
By its nature, the forces between the two chains varies considerably depending on the position of the sled. You are trying to move the highest and lowest force positions to beyond your cutting area. This will help with accuracy of the machine.
The actual dimensions you make are not super critical as the machine will basically measure itself during the calibration procedure.