grammar - is 随分 of a higher degree than なかなか (with positive verb) ?

my dictionary shows both 随分 and なかなか (with positive verb) as "very/considerably"

I was wondering is it true that 随分 is of a higher degree than なかなか?

Like あんたなかなか勇敢だな。= 75%

and あんた随分勇敢だな。 = 85%?

Answer

Yes. なかなか means mildly, moderately. The origin is 中中, 'middle-middle'. It is lower in degree than 随分.

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Below is slightly technical.

Elaborating on rintaun's point, なかなか has another usage: used with a negative expression. There are words that can be only used within negative environments (and some other environments, subsumed under what are technically called downward entailment environments).

決して 食べない [Japanese example]
全然 食べない [Japanese example]
おかず しか 食べない [Japanese example]
He does not study at all [English example]
I am not paying a red cent [English example]

Sometimes, a word can be used either under this usage or under a different usage:

He cannot eat any-thing. [Negative usage]
He can eat any-thing. [Different usage]

Depending on the usage, the scale differs. なかなか is an instance of such word. It has a usage that requires a negative environment.

彼は なかなか 食べない [Negative usage]
彼は なかなか よく食べる [Different usage]

In this negative usage, the degree of なかなか is stronger.