I want to calculate relative power of two signals what is the correct matlab
code to do that. Actually one signal is the filtered version of the other,
because i want to find the power contained in the specific band of
frequencies.

The code I am trying to use is this:

x = wavread('sc4012_subj1_Pz-Oz.wav');

%Filter Specs

n;

Wn=[8 12]/50;

[b,a] = butter(n,Wn,'bandpass');

x_fil = filter(b,a,x);

%%%% Welch's Periodogram

%[Pxx,f] = pwelch(x,window,noverlap,nfft,fs)

[X,f] = pwelch(x,200,100,[],100); % EEG Power spectrum

[X_fil,fa] = pwelch(x_fil,200,100,[],100); % alpha power spectrum

%%%% IFFT

X_FIL = ifft(X-fil);

X = ifft(X);

%%%% Cyclic Shift of IFFT results

Nhalfa = floor(length(X_FIL)/2);

SecondHalfa = X_FIL(1:Nhalfa+1);

FirstHalfa = X_FIL(Nhalfa+2:length(X_FIL));

afixed_cyc = [FirstHalfa;SecondHalfa]; % EEG cyclic

Nhalfp = floor(length(X)/2);

SecondHalfp = X(1:Nhalfp+1);

FirstHalfp = X(Nhalfp+2:length(X));

pfixed_cyc = [FirstHalfp;SecondHalfp]; % Alpha cyclic

%%%% Finding Alpha Relative

alphaRelative = afixed_cyc./pfixed_cyc;