Grammar Corner: Further versus Farther

I was advised recently by my MFA mentor to look into the difference between these terms.  While I'm not sure if my misuse stems from misinformation or from a repetitious typo (I somehow always type breath when I mean to type breathe), I followed my mentor's advice and here is what I've found.

It's confusing to say the least as one appears to speak of actual physical distance while the other does not.  But if we look at the further, freedictionary.com tells us this:

fur·ther (fûrr)
adj. A comparative of far.
1. More distant in degree, time, or space: a result that was further from our expectations than last time; the further lamppost.
2. Additional: a further example; a further delay.
1. To a greater extent; more: considered further the consequences of her actions.
2. In addition; furthermore: He stated further that he would not cooperate with the committee.
3. At or to a more distant or advanced point: went only three miles further; reading five pages further tonight. 

So when it comes to further we might say: Jimmy has come a great deal further with his studies than little Janie.  We might also say: I want to take my game further than it's ever been before.  In the previous sentence, 'further' indicates an intangible destination.  But what about concepts that two possible interpretations such as heaven/nirvana/the ether?  "I wanted to go to heaven but knew I would have to go much further to get there," versus, "I wanted to go to heaven but knew I would have to go much farther to get there."  Does one mean spiritual distance and the other mean down the road?  That's where it gets confusing for me. 

far·ther (färr)
adv. A comparative of far.
1. To or at a more distant or remote point: ran farther than the others.
2. To or at a more advanced point or stage: I went no farther that day.
3. Usage Problem To a greater extent or degree: carried the idea farther.
adj. A comparative of far.
More distant; remoter: the farther shore.

Methinks that the free dictionary might not be enough to answer this grammar question.  I've called in some experts, namely Grammar Girl.  

In this article, she breaks it down simply into:

Farther = Far(ness) as in the physical distance ('far' being a pnuemonic device)

Further = Figurative distance, which can include abstracts like degrees

This is satisfactory for me, but now that I've focused on these words, I feel like the simple breakdown might be too simple.  English has a knack for breaking all of it's own rules, after all.  But for now, I'll try to keep farther down the road and further up the scale.  And when it comes to 'death' or 'heaven' or 'hell,' I suppose it depends on the meaning the author is going for.  "I wanted to go to Heaven, Mississippi but it was farther than I realized," or, "I wanted to be a good girl and go to heaven but preacher said I was further into sinning than a Saint into praying."