One of the most important aspects to taking accurate and meaningful measurements is placing your measurement mics correctly.
The book, Meyer Sound Design Reference explains starting on page 212 and going to page 217 covers the subject:
Here are the basics.
There are (3) types of measurement positions.
“The key to mic placement is to use positions that provide a global representation with a minimum of unique local conditions”
If you only have (1) mic and a single chance for making a measurement to judge how your sound system is behaving and also to decide how to adjust it for a linear (flat) frequency response, you want to place your mic on axis of the main speaker / array, etc…
Ideally you want to measure only one speaker at a time until you have verified each speaker / amp & speaker chain is working properly. There is little purpose in measuring an entire PA at once (as people do with an RTA) if you have no idea what speaker is producing what signal.
Are the subs working? Are they part of the main PA or on an AUX send? Are the underbalcony speakers being fed off the L/R matrix or a different send?
It all depends on how the PA was put together. If it’s your PA and you know everything is working, how did you verify this? Without any measurement equipment or experience, our ears can tell us whether we have full range sound if we know what to listen for.
I’m straying from the “where to put your measurement mics” answer but it’s worth making this very clear. Due the nature of a complicated system and the human factor, many PA systems have something wired wrong. It could be between the amp and the speaker or between the mixer and the DSP device. You could have a speaker cabinet that had a driver replaced and the wires inside the cabinet were hooked back up incorrectly.
How would you know?
Let’s take the example just given. Let’s say you have a (2) way speaker with a passive crossover. This means that unless the cabinet has been opened and a speaker replaced, the speaker itself should be optimized from the factory for it’s best performance (based on size, cost, materials used, design of cabinet, design of crossover, etc…)