Low.pass filter

Hello everybody.
As I am finishing up my project of customizing EEG-SMT I encountered some
things I can't seem to understand about the OpenEEG. One of those things is
the 3rd order besselworth filter which I cannot successfully
reverse-engineer in several Bessel filter calculators. Every time when I
enter the provided values of cutoff frequency, order... I get really
different values of the parts used. It is really important to me,  I'd like
to set it to 49Hz cutoff because of european electric network of 50Hz.
That's why I get too much 50Hz ripple. I will share the schematics, board
and electrode designs, but at first I need to understand the filter. Is
there someone, who will be able to explain the value selection and formulas
used for this filter? I'd be really thankful. I spent hours maybe days
figuring it out, but I can't seem to understand it. I used multiple
calculators, but this one looks the best:
http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html

Thank you in advance.
--
Damián Filo

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