I have a studfinder, a bosch gms120 but it's readout is inconsistent, the place I'd like to screw is about 1.5m from a light and it seems to think there is a livewire everywhere. Would I need to go into the loft to mark out where the joists are? can i just knock on the ceiling and accurately find one? is there a common logic to wire placement leading to a light or should I try and find where the wire leads? or should the bosch be working and I'm just using it wrong?