orthography - When to drop the 'e' when ending in -able?

I've seen a thread that generally asks about Creating words with “-able” suffix But I don't think it answers my point, though they are admittedly dangerously close topics.

When do you drop the 'e' when forming words suffixed with -able. My Spell checker likes Unforgivable but dislikes Forgivable. Dropping the 'e' in the first case, and adding it in the second makes my spell checker happy. How do you determine when one is ok? Note that this is different from the linked question, where neither with or without the 'e' is accepted.

_{(Having checked the OED it seems there is one accepted spelling of Forgivable but two of Unforgiv(e)able)}

Answer

The only situation that comes to mind where an -e- is absolutely required before -able is when it modifies the pronunciation of a consonant, typically g or c:

Manageable (g as in giant) versus
**managable* (g as in gut)

Traceable (c as in once) versus
**tracable* (c as in cut)

Of course, that problem would exist in reverse for -ible words, but in practice it doesn’t arise as these are less common than their -able cousins.

The -e- also serves to make a vowel long where otherwise it could be interpreted as short. Scrapeable definitely begins with scrape, scrappable definitely begins with scrap, but scrapable could go either way; and if the common misspellings of short-vowelled words ending in -able are any evidence, it is likely to be read as scrappable.

In addition, when adding -able to words that end with a syllabic consonant, the -e- tends to be retained, to stress that the consonant still comprises its own syllable. tchrist offers:

Throttleable, (un)settleable, (un)whistleable, (un)riddleable

To my mind, whistlable is three syllables—[wɪs.lə.bɫ]—whereas whistleable is four: [wɪs.l.ə.bɫ]. That says nothing about which one I’d choose, because my pronunciation varies freely between them. Although, I do think that a two-syllable pronunciation of “settlers” (thus a three-syllable “settlable”) sounds rather Southern or Southwestern.