Look at the thread
"5's PRO + FSL Setup" from Wendler forum, November '15. Kingbrady asks Jim to verify the percentages and the progression.
If you jump into Starting Strength with super light weights, and do 3 sets of 5, adding 5 pounds each session, you're going to spend awhile lifting too light. At some point, you'll add enough weight, so your sets are "just right." But then you'll quickly add weight, trying to progress. You'll race through the productive training weights, and soon be training too heavy. All your beautiful smooth lifts degrade into ugly grinds.
The 5's progression is set up to keep you in that productive training zone. It's not too light to get a good workout. Its not too heavy to do the lifts properly. Its not so much volume that it's impossible to recover from.
It will allow you to "switch up" your training, without completely abandoning the work you've done up to this point.
So you can run the 5's Pro for 2 cycles, or 6 weeks. Then deload for a week.
Save a couple bucks a week during this 7 week period.
At that point, resume "regular 5/3/1"
Hopefully, the Jim's new book will be out by then.