Nog when you moving the reciever about leave it in each place for 4 or 5 secs once you got a close mark,find the lowest reading then move it a foot up,down,left,right but leave it on the floor in each position for a bit each time,if you get a lower reading when you move it start again using that point as the centre,like you say sometimes you can move it a few feet without it altering because of dips etc,if that happens find the centre of that mark and dig there,thats what i do and always dropped into the tube so far