Right, not that black and white. If I had 15 years in year 1 = $150,000, I would give about $10k each of the next 15 years. If the investments dropped by 1/3 I would give $7k each of the next 15 years. Diversified investments tend not to go down and stay down forever, so the exact amount would fluctuate over time. Also, there's no reason to do this except to claim the $150k deduction in a specific year. DAFs have higher fees than low cost index funds in taxable accounts so I don't push money there for no reason.
It can go the other way, too. I opened my DAF in 2006 or so and left it all in cash until February 2009. Then I moved it all to stocks. So now charities are getting twice as much as I originally planned to give. Should I not do that?
I *really* like DAFs for their cash flow predictability. Because of the way I do mental accounting, I make donations every 2nd or 3rd year from my retirement portfolio - not from my paychecks. And I try to do it after strong runs up in the market (I suppose the counterpart to livesoft's 'really bad day' would be a 'really good month.' The advantage is that at the end of the calendar year, just when I'm gearing up to fund Roth IRAs and start paying back into SS and our 403bs in January, and paying off holiday bills and holiday airfare - I don't have to pay for a deduction out of cash flow (my paychecks). And of course I avoid the capital gains taxes on appreciated securities, something that is hard for small charities to handle at times.