What is the slope of a transfer function?
Okay. I think i got it after some clarification from Dan. Thanks Dan. !

I think what i was confused with was the step size in my frequency vector.. i had thought to have the final result in db/mhz.. the steps should be in 1 Mhz.. but i now realize that the freq vector could be in whatever steps i like but if its in Hz then i need to have my final answer normalized by 1e6 to get from  db/Hz --> db/Mhz

So this is the code i finally used..

fs_ADC=200e6;
N=2^11;
step_size=fs_ADC / N ;
f = 0: step_size: fs_ADC;
H = fir1(f);  
H_db=20*log10(abs(H));
slope=1e6*diff(H_db)./diff(f);  % db/Hz--&gt; db/mhz
plot(f(1:end-1)/1e6,slope) 


Figure result till 30 MHz is shown below: