There are some RCs that allow you to set a curtain or cut-off frequency (Dirac, Aud XT32, for example).
Common practice is to do so just above your room's Schroeder Frequency, usually somewhere between ~100-200Hz... (though I have seen different numbers ranging slightly higher rather than lower)... this being the point where your room stops acting as a resonator and the frequency will vary by room.
A cheaters way to see the Schroeder Frequency is to look at your in-room measurement, uncorrected, and judge where the peaks and dips begin to smooth out.
Of course, some people just say "set the curtain at 500 and call it a day."
Anyway... You don't want to cover everything in your room. IIRC, about 25% is all that is needed in most cases.
Treatments should be based on your room measurements and be placed strategically.
Why would decoupling the room create a more reflective surface?