[Ifeffit] E0 and background subtraction

Hi everyone, I have the following question: I have a set of spectra collected in transmission with 3 ionization chambers and a standard between 2nd and 3rd ion chambers for calibration, so I know my spectra are nicely calibrated relative to each other. Should I then force Athena(/autobk) to use the same E0 for all of them (e.g. use the atomic value) or let it find E0 independently for each sample? Thanks! Vadim.