GTEABJ Question

In John May's book Get The Edge At Blackjack, I came across the following passage:

With a 100 per cent front loader, i.e., a dealer who exposes the value of every card , the maximum possible edge is the 10 percent obtainable from hole card play.

I was always under the impression the maximum edge was higher, around 13%. John, where did you get that number from?