"Reasonably" is an adverb, so it must describe "how" a verb is being performed. E.g. "He acted reasonably". He acted, yes, but the way he acted was "reasonably". "A reasonable-enough assumption" (I've hyphenated "reasonable-enough" to illustrate that it's a compound adjective, although it's rarely if ever written that way, sadly) is another way of saying "an assumption that is sufficiently reasonable." "Reasonable" is an adjective, so it modifies (gives more information about, or specializes) a noun.

I believe Steve is right on the money here. One thing, I've noticed is that adverbs are not taught very well. Usually, though not always, adverbs are to show degrees or extremes as well as modifying verbs. Your house is big. My friend's house is small. Your house is so big. My friend's house is very small. He acted reasonably. Modifying verb; to act She was (unreasonable) in her demands. Adjective form; showing condition She was unreasonably calm, given the severity of the situation. The weather seemed unreasonably hot for a New York winter day. He did a reasonably good job, given the circumstances and stress he was under. But if it is just a condition; then, the adjective form applies: It seems a reasonable (enough) way to do business.