Before you even think about buying any treatment you need to
consider how your room is set up as this will have a big effect on the
accuracy of your monitoring.
Your monitors should be firing sound down the length of your room and
they should be angled in to create an equilateral triangle (all 3
sides are the same length). Set them up so the tweeters are pointed
directly at your ears and isolate them using
Sit yourself centrally between the left and right walls, but about 4
10ths of the room length from the wall you are facing. That way your
monitors should not be too close to the wall and you will not be in
the middle of the room (which is often a dead spot for bass).
Don't worry too much about getting it perfect, you can tweak the
position when all your treatment is in.