Ernie Parker says to not use any weighted ball that goes over a 20% overload. I will go by Ernies experience on that call.
Cheri Kempf's book "The Softball Pitching Edge" which I believe the OP is referring to, also states to not to go over or under by more than 20% or you risk injury. Overloading/underloading programs are used in many sports to produce speed increases and power. These programs should be undertaken with the guidance of a knowledgable instructor if you want to get the most out of it.
I will disagree with that on the underloading issue. I use Wiffel balls to teach finger finesse and pinpoint accuracy. NEVER had a student report any injuries from pitching.
Then we go long toss with the step drill and from normal full pitch for 20 or so pitches at about 50-55 ft. Then we get into our pitching session.