Need help w/ 7.1 speaker placement (first post)

Hi,
I have an Onkyo 7.1 HTIB. We moved recently, and the living room already has a TV built in to the wall with a stereo sound system. So I decided to put our 52" tv in the den and set up the 7.1 htib system there.

I'm having a bit of trouble deciding what to do with the rear and side surround speakers though. I included pictures of what I thought I would do. Keep in mind the wall to the right hand side of my desk is actually a sliding glass door with about 8" of dry wall lining the top.

Before I moved, I had the rear speakers mounted on stands that only came up to ear level when seated. I'm trying to wall mount all my surround speakers this time and was planning on tilting all the speakers 30 degrees down.

Before I drilled holes on the wall I wanted to make sure that the speaker placement isn't too high.... The side speakers can be adjusted lower, but rear speakers can't because of the sliding doors.

Also, I'm a little concerned on whether or not the rear speakers are too far from my seats compared to the side speakers.

Any help would be much appreciated. Oh and the face of the front speakers are on level with the face of my bookcases unlike the rough design I drew. Thank you

Any help guys?
The speakers are just lying on the floor waiting to be mounted.
I looked through the forums for similar issues, but there seemed to be conflicting opinions on how high is too high for surround speakers.

Thanks again in advance

Thank you for the response.
From the rear, I will be about 4'8" away. I was actually a little confused on how far apart the rear speakers needed to be from each other.
I was thinking about a sofa length (depicted in the picture) apart.

That made me question whether or not I should point the rear speakers around 10 degrees towards the center of the sofa (or whether or not it even mattered).

The system is a HT-S6100, and I think I'm mostly going to be listening with Dolby Pro Logic IIx.

Thanks a bunch for your response.
Would you tilt the speakers towards the middle of the sofa as well?

Thanks again for your help!