I measure low freq and hi freq in two different ways. First, move your listening chair or couch out of the way, but mark on the floor where your ears are when you're sitting in it.
Low freq:
Mount the SPL on a short (table-top) tripod adjusted so its mic is between 1 and 2 feet above the floor over the listening position, with the SPL mic pointed toward the ceiling, like Kal says.
Hi freq. (350 Hz (+/-) and above:
Mount the SPL meter on a normal tripod adjusted so its mic is at seated ear level (about 40") over the listening position and pointed forward between the speakers. You may have to rotate the tripod head 90 deg. sideways so you can read the meter from a few feet away without standing on a ladder ;-))
