Have a look at this
http://www.4seating.com/audio_DolbyReference.htm#s3.2
For what ever reason Dolby removed this detailed document from their website a number of years ago. Maybe thay had decided to encode new material with a differnet soundstage, or maybe it was in response to manufacturers who wanted to sell speaker on stands. Its a lot easier to balance a speaker on a trendy 1m tall stand than a 1.8m to 2.2m tall one.
As you are wall mounting the speakers you should be able to obtain the 2 to 3 feet above ear level detailed in the document.