I've done two such tests in my basement.

first test was hung from the ceiling about three feet away from the steps to
go upstairs


Second test was hung 6 ft away from the steps and from a string at a height
of 5'10'' from the floor


BOTH tests proved the same levels +-2

My radon was high and I have to mitigation......above 20 ....That was with
the basement closed off....Now my basement is unfinished and OLD my
foundation is STONE!

SD