[exim] ''multidomain'' DKIM: sender or from?

I'm testing DKIM implementation in my exim server, and looking out google
point me here:

    https://debian-administration.org/article/718/DKIM-signing_outgoing_mail_with_exim4

for multidomain setup (i'm interested in) the author proposed:

    DKIM_DOMAIN = ${lc:${domain:$h_from:}}

but i think it is better to use the sender, eg do:

    DKIM_DOMAIN = ${lc:${domain:$sender_address}}

or not? reading https://www.ietf.org/rfc/rfc4871.txt, point 5.5, seems that
From: is a signed header while the sender (Return-Path:) not.


I'm a bit confused... thanks...

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