Quickie mirror point question - on the trap or on the wall?

Hi there Choosing where to put my side wall reflectors today - and I've forgotten...... When using the mirror technique should I use the mirror pressed up against the wall or should I space it from the wall so that it is in the position of where the front of the panel would be (in my case 17cm away from the wall)? Makes quite a difference so I want to make sure I get it right!!!! Cheers Max

put it on the wall is simplest to keep steady and mark. the absorber will be much larger than that single spot.