Thermostatic Shower Install

[Le Rouret]

Hello...
I have a Hudson Reed shower to install but don’t get this...see picture below..
The problem is once I do up the compression joints then the large O rings will not be flush against the wall...there is about two millimeters of distance between O ring on the large nuts and the wall..

 

[Ben-gee]

Set the brass fitting further into the wall then?
Additionally they can easily be cut down in length if that is required.