Ok, so I have spent some time trying to find so explanations of the grammar used in this sentence but I can't seem to quite understand what the 「自分でも」 and 「くらい」 are doing in this sentence.
「自分でも不思議なくらいに落ち着いた心で、目の前の美少女に静かに尋ねた」
My best translation of the sentence is: "with a strangely calm heart, I quietly asked the beautiful girl before my eyes."
here's some of sentences before to give some context
俺は、頭で教えるよりも先にその子を突き飛ばしていた。 そして・・。 ・・自分でも不思議なくらいに落ち着いた心で、目の前の美少女に静かに尋ねた。
with a strangely composed heart, I calmly asked the beautiful girl before my eyes
自分でも : oneself / even I .
It was strange even for himself, since he thought he shouldn't be feel that composed.
不思議なくらい : strangely / oddly enough / to the extent of being strange
Just 不思議 means strange/mysterious, adding くらい modify the meaning into above explanations
It's a common rhetorical phrase, though not idiomatic, to express total unexpectedness.
自分でも（不思議な／びっくりする／驚く／よくわからない etc.）（ほど／くらい）
It tells that what you did or felt was out of your own expectation. You could translate it like "so — that even myself don't (didn't) know why".
