The cut grading criteria used in that particular book are not very widely accepted but I think you are correct that he does not recommend buying stones of over 61% depth. No, I don’t agree with this advice.
Cut grading of diamonds, especially round brilliant cut diamonds, is a subject of great interest here. You’ll find it discussed at great length in both the tutorials at the top of the page and previous posts in this forum. As you point out, not everyone uses the terms the same way and it causes a fair amount of confusion. A class I- II – III system was used by GIA a few decades ago but wasn’t really specific enough to prove to be useful. It was was subdivided by AGA into Ia, Ib, etc. and the AGA system enjoyed, and continues to enjoy fairly broad acceptance. It’s explained nicely at www.gemappraisers.com under the DIY cut grading section. Buzzf's book has it’s own system using similar terms that I think are explained in the book. There is a brand new system from GIA that is being hotly debated and can be viewed at www.facetware.gia.edu that grades stones on a 5 step scale from poor to excellent. AGS grades stones from 0 –11 and AGS-0 is a popularly used definition of ‘ideal’ although they changed this definition last year so really there is a pre-2005 AGA and the current AGS approach.
What’s a customer to do?
Happy hunting.