My modest cellar bunker is little by little getting ready for use, and it seems it's not at all as bad acoustically as my worst fears were.
Now, is it a waste of time to run (was it white or pink?) noise through the system, and record it with a Zoom H2 in various spots in the room, and then study the curves with Span or similar? It would, I assume, point the worst spots of the room, maybe the best as well, but with what kind of reliability/accuracy?
Buying a dedicated measuring mic/software is not an option, but I could use my AKG 3000 or Rode M3  recording the noise with a laptop, if you think that would be better. 
 
As I mentioned, the room sounds surprisingly decent in the most important spots, but I'd like get some confirmation to what I hear.

Check this out.

 
You might want to play around with Room EQ Wizard.  I'm sure you can get useful results for a mic you already have.  The software is free and will tell you just about everything about your room.
 
Hang out a bit at https://www.gearslutz.com/board/studio-building-acoustics/ to learn what's useful and affordable, and what's over the top.  One of the stickies describes how to measure a room.
 
The quality of what you hear varies greatly depending on where place your monitors and your ears, as you seem to have noticed.  The first stage rule of thumb is to face along the length of the room, in the center width-wise, 3/8 of the way back from the front/end wall.  Monitors in an equilateral triangle as far from the side walls as you can get them.  You want symmetry, so it wouldn't be good to have a huge opening in your left wall and not on your right.
 
So what are the dimensions and characteristics of your room ?  Surface materials, etc.
 
 

ARC2 is the way to go if you're willing to splash the cash. I got it in a sale last year and have never looked back, the difference it makes is amazing.

There's actually not much point in describing the room, as there are no alternatives to the positioning of things. I'd just want to find out how good or bad it is the way it is now. If it's bad, I'll just have to live with it. But anyway:
 
The room in total is 7m x 5m but it's split at 3,5 m to separate the other half for storage room. The closet-wall splitting the rooom is filled with clothes in hangers. So the music-hobby half is about 3,5m x 5m, work spot is in the middle of the short wall. I can roll the table and monitor speakers about 1m further from the wall when I start, say, mixing, which puts me sitting approximately in the middle of the room. It's partly of light gravel blocks, partly concrete, only 2,05m high.
I've added absorbing panels (2 cm thick) on the ceiling and covered the walls with the same panels. I don't know what that material is called in english, could be "wood fibre panel". Magazines from the 30's-40's, paperbacks from the fifties and comic books from the sixties cover the ceiling corners around the room, all walls have unsymmetrical shelves with books, CD's etc. and two sofas are in the back.
 
As mentioned, it sounds surprisingly decent, but I'm interested in mapping the less obvious weaknesses.

i use ARC2.
 
before arc = bleh
 
after arc = ahh

Room EQ Wizard is the place to start - not ARC.
 

either way, the end result is what matters.
 
use SOMETHING! 
 
 
lol
 
(preferably room treatment first)
 
and to say that arc isn't a place to start, is a bit head in the sand.
it worked for me.
 

It sounds to me like you have a pretty good space with absorbing materials scattered throughout. If you want the budget way around it, sit in the spot you most likely will be monitoring from, run some sample sine waves through your monitors at various frequencies and listen. Then slowly move around the room listening for dropouts in volume and loud spots. These are your problem areas from standing waves and reflectins that are in phase with each other. You can mark these areas off with tape, so you know where they are. As long as you are aware of them, and avoid monitoring from them, you should be all right. I know for me, just knowing where they are helps me avoid them. One of my dropouts around 200hz occurs right in front of my computer keyboard. After I tweak, I back up about a foot and sit tall to monitor, so I know the room isn't affecting what I hear.

Thanks for your input, friends.
 
That  Bitflipper link by Gustabo and EQWizard will be the routes I'll go. ARC is not in the picture.
And as there's practically no room for further room treatment, I'll just have to know the weaknesses and then live with them.