If you think this "に" can be changed into ”を" or "で".
ってくらいを考えておいて → no problem in Japanese.
ってくらいで考えておいて → no problem in Japanese.
本を読むと幸せを感じる → no problem in Japanese.
本を読むと幸せで感じる → You don't use like this in Japanese.
くらいにかんがえておいて is like similar to idiom-ish way of using Japanese.
It is very difficult to explain the subtle difference between your mentioning case.
We Japanese are using this ってくらいにかんがえておいて phrase without much consideration.