As Brian Shaw mentions, the Dunn and Wakefield book is an excellent guide. Here is an excerpt on incident metering and the use of flat disc vs hemispherical (Norwood style) diffusers with incident meters. (The Exposure Manual, 4th edition, page 131, column 1)
What's implied here is that the Duplex method is mainly for flat-receptor incident meters, and that domed incident meters compensate in the same way as the flat-receptor Duplex method until the light source is more than about 40 degrees behind the plane of the subject.
I have the third edition of this book, and it is a really great reference.
Mark Barendt, Beaverton, OR
"We do not see things the way they are. We see things the way we are." Ana´s Nin