I agree with your conclusion: the only way to grow Pf is to grow the economy.  I'm personally not interested in growing Pf, but I do want to see the economy grow.  I would rather this happen with inflation: grow the market cap / total value store with an increasing number of coins and keep the Pf stable, but that's really a topic for altcoins.

Yes, the B and D terms in my original formula are a little fuzzy, since they incorporate several different elements in a nontransparent way.  It makes it easier to initially understand at the expense of being messy when you want to apply it in detail.  I think breaking them out is worthwhile.

Where does S belong, in D or A*B?  Which way is right?  Well, what are savings?  An attempt to store value?  A casual investment in the future?  Avoidance of friction?  I don't have a simple answer to that.

In the current market I don't think anyone has savings that last more than a week.  I think people who hold their coins longer than that pretty universally consider them an investment. Thus, I rolled S into A*B - the coins are still in circulation and intended to be spent on goods even if those goods are not yet selected.  At present, that's just taking a slightly broader picture of required value store.  In the future, I think you're right to roll it into D.  In practical terms, it works either way: in A*B it slightly stabilizes Pf; in D it tracks the market quicker.

I'm not sure why you put a + between the S and I.  Here are the two ways I'd write it:

Pf = V * T / (A - S - I - L)
Pf = V * T / (A - (S + I + L) )

A bit more background for those who didn't read the original thread:
Much more detail on how this model works: https://bitcointalk.org/index.php?topic=42514.msg579137#msg579137
Explanations for my guesses for the variables: https://bitcointalk.org/index.php?topic=42514.msg579196#msg579196