[Ifeffit] E0 issues

[Hana]

I guess my question is obvious to most of you, but after some practice and 
reading, the following is still unclear to me (hopefully some other beginners 
will benefit): Normally I have calibrated my spectra using a calibration foil – 
so energy shift was done according to that companion standard. Doing a simple 
linear combination fit, I have set all spectra to the same E0 value, somewhere
on the edge, above the first derivative maxima (was it a good practice?).
Recently, I found myself actually confused about E0; since my samples inherently 
have a phase difference (which is the base to my ability to differentiate them), 
how a certain reference point on the spectrum can be determined? What can be
done when I do not have the calibration foil (especially for these heavy
elements that do not have specific sharp feature)? And further, now that I am 
starting to work on the structural model; how actually IFEFFIT determined the 
energy shift when there is no specific reference point? 
I would be happy to get a better insight.