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)}

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.

When I was in elementary school I was taught that the rule was: If a word ends with a silent "e" that serves to make the previous vowel long, and you add a suffix that begins with a vowel, drop the "e". If the suffix begins with a consonant, do not drop the "e".

Thus: forgive + able = forgivable

But: forgive + ness = forgiveness

Here's an Internet source that states this rule like this, "Another thing to keep in mind is that when a suffix is added that begins with a vowel and the main words ends with a silent "e", that silent "e" should be dropped like it is in the words pricing and surprising. However, if the suffix used begins with a consonant, the silent "e" should be kept such as in the words likeness and advancement." [http://www.thefreelibrary.com/English+Spelling+Rules+Adding+Prefixes+and+Suffixes-a01073900597]

thefreedictionary.com gives the spellings as "forgivable" and "unforgivable" -- no "e" in either word.

But frankly I often see spellings that do not follow this rule. Whether this is because people are not following a consistent rule or the rule as I was taught was a simplification and there are other cases, I can't say.

By the way, another general rule I learned is that when adding a suffix that begins with a vowel, if the base word ends with a short vowel followed by a single consonant, double the consonsant. If the vowel is long or there is more than one consonant, don't double the consonant.

For example, big + er = bigger. But cool + er = cooler (the vowel sound is long) and bank + er = banker (two consonants). (Ooh, I just noticed that I am using the suffix -er with two different meanings: "bigger" means "more big", but "banker" does not mean "more bank". In "cooler" it can have either meaning: more cool, as in "Today is coooler than yesterday", or a thing that cools, as in, "Put the soft drinks in the cooler.")

Definitely, there is no single rule or even a set of rules that can be applied consistently and algorithmically in all cases.

At least the set (un)forgiv(e)able seems to have started the process of shedding the e c1880. I do not yet know for sure what happened around 1880, though.

nGram forgivable | forgiveable | unforgivable | unforgiveable 1800-2000

What is surprising to note, however, is that the e does still survive, apparently fighting back.

nGram forgiveable | unforgiveable 1960-2000