It says this in variables.conf at http://files.directadmin.com/service..._spam_fighter/ under "Files":

-- variables.conf

If you want to customize the file, create your own file:
-- variables.conf.custom, and set only the values in this file as desired, and they'll override the defaults.

However if you do that, Exim will stop and these kinds of errors will appear.
Starting exim: 2017-08-27 19:01:12 Exim configuration error:
macro "EASY_LIMIT" is already defined (use "==" if you want to redefine it
So it seems you either do have to adjust the exim.variables.conf file or use == instead of just create our own exim.variables.conf.custom with the wanted value?
If this is correct, the help section needs to be updated. I did not test with == yet.