Publishers tend to be consistent with recipes and one of my older (1995) Storey Publishing books mentions 27 "gravity points" from a pound of base malt.
As a double check, I was able to preview portions of the book. The Alaskan Amber recipe is 5 gal, OG 54 with 3.5 lbs DME and 4.25 lbs base malt:
So, assuming no typos and no math errors, 75% is a good starting.