Error on TFractionFitter

Hello all,

I am using TFractionfitter for estimating the signal fraction in a certain
distribution. I use two templates:
mc1 - signal
mc2 - bkg

The fitting range is 0-0.04. Aim is to determine the signal fraction within the range 0-0.013
The way I do is:
=====Method A=====
(a) Get the fraction of signal and bkg which is reported by TFractionFitter
(b) Use the input template histograms to calculate the signal fraction and bkg fraction within
the specified range.

======Method B======
I checked that if I use the MC template histograms which are got by doing:
GetMCPrediction(0) and GetMCPrediction(1)
for calculating the fraction within the specified range, then I get different results
from Method A.

My question is:
Is it valid to use Method A and then use the difference between Method A and Method B as the
error on template method?

Thanks & Regards,