Adverb end in long or short e

I’m getting confused. Is the final e in adverbs long or short. Or is there some sort of way to work out which are long and which are short. (Maybe this has been discussed before, but I can’t find it.
rectē but bene etc
Thanks
Jim

I think you just have to learn it.

Allen and Greenough has:

"604. The Quantity of Final Syllables is as follows.

e. Final e is short.

nūbĕ
dūcitĕ
saepĕ

Exceptions—

Final e is long:

  1. In adverbs formed from adjectives of the first and second declension, with others of like form.

altē
longē
miserē
apertē
saepissimē
(So ferē and fermē)

But it is short in benĕ, malĕ; īnfernĕ, supernĕ."

http://dcc.dickinson.edu/grammar/latin/quantity-final-syllables

I found this in Adler
1 Adverbs in ē denote a quality, and are formed from adjectives and participles of the first and second declension.
Altē, high; latē wide; longē long, far; liberē freely, miserē, apertē, saepissimē. doctē learnedly; But benĕ, malĕ; īnfernĕ, supernĕ
2 Adverbs in um and ĕ are derived from neuter adjectives of the second and third declension, without any change of form. Impune, sublime, facile, difficile (faciliter, difficiliter)
3 Adverbs in iter denote manner from adjectives and participles of the third declensions. Adler 616
But adverbs that relate to the second declension could still go either way. So memorizing or familiarizing is the only way. Jacobulus

Fortunately the superlatives ending in issimē (etc) have a long ē (acc to Wheeloch).