Sergio and Fabio,

I like to encourage you to use STEREO EUVI data, which would provide
you the ultimate test to subtract the correct background, because you
see a loop from two different directions and have two independent
backgrounds. You can test the self-consistence of background subtraction,
if you obtain the same EM of the loop from two spacecraft. 
If you look into my recent STEREO paper, you see that one can
subtract the background with an accuracy of about 10%.
See Fig.8 therein.

Cheers,
Markus

 Aschwanden,M.J., Nitta,N.V., Wuelser,J.P., and Lemen,J.R. 2008, The Astrophysical Journal, 680, 1477-1495
URL1="http://www.lmsal.com/~aschwand/eprints/2008_stereo2.pdf" 
First 3D reconstructions of coronal loops with the STEREO A and B spacecraft: II. Electron Density and Temperature Measurements