Sonar Height Placement and Strength

Hi, I’ve just taken a look at this thread, but there was still a question left unanswered there:
how high up should the phone be? My nightstand is around 10cm shorter than my bed, so my phone is lower than my body.
Would change the height of the phone to my bed level or higher benefit the readings?

The current readings have been quite mixed: sometimes I get a lot of deep sleep (92% - the activity graph is basically empty) while other times almost none (7% - the activity graph is all over the place).

Also, I’m completely unsure about the sonar settings I should use. Should I be able to hear the ultrasound or should it just be at a low volume? I am currently using F19 with the volume bar around 25%, but the signal strength is very low (actually right now it is saying like 1 or 2 strength, but when I first setup it, it was around 20). I have the OnePlus 3T.

Also, not sure if that matters, but I’m 17 years old.

Thanks in advance,
Diogo