How to place speaker for Atmos with a old TrueHD 7.1 reciver?

Hi

I run a HTPC with bitstream output on HDMI. My old receiver (onkyo 606) takes TrueHD on 7.1 channels.

If / when I want to bitstream Atmos, Should I reposition my speakers to a 5.1.2 setup, or shall I still use the 7.1 setup?

I have understood that Atmos is using the TrueHD on a 7.1 setup, and extra bits are to be used for the exact placement of objects. I can, as my receiver cant decode Atmos, use the extra bits. That’s fine. But are the 7.1 "base layers" Atmos are decoded in made for a soulotion with celing speakers, or the old traditional 7.1 setup? I am reading some confusing (and sometime contradiction) posts about this...

So the basic question is; How to set up Atmos on a 8 channel TrueHD (but not Atmos) system?..

BR /