This question is on the following sentence, from chapter 7 of La porte étroite by André Gide.
Sa voix restait si calme et naturelle que mon exclamation m’en parut d’autant plus ridiculement emphatique.
I am trying to account for en and have come up with the following ideas. Please let me know if they are right. Where they are wrong, please tell me what I should have thought instead.
A d’autant plus + adverb construction may be completed with either a pour-phrase or a que-clause supplying a cause, reason or occasion. (This would be rather like so much the more + adverb completing with a for, now that or as.)
En in the Gide sentence results from (represents or means) pour + le. From this I should draw the lesson that en may unpack into pour + something, and not always de + something.
Le, or the 'thing' in en, in this case would be the whole preceding clause, i.e. Sa voix restait si calme et naturelle.