Hi what is the optimal postition to place the M5 in to get optimal coverage? 

I've got 3 Deco M5 in my home and the coverage isn't quite cutting the mustard.  

We've ordered another 3-pack to address this, and am wondering whethe it will be worthwhile to stick a couple of them either on the ceiling (plasterboard), on walls (double brick/plaster), or even in the roofspace, to get better coverage across the whole space (5 bedroom, 275sqm brick and tile).