Tom,
I think there is value in understanding your withdrawals in terms of both initial portfolio value and current portfolio value.
When using only the initial portfolio value, withdrawal rates will always rise through time. You increase the withdrawal amount with inflation over time while holding the portfolio value steady at the beginning value.
When using current portfolio values, both withdrawal amounts and portfolio values (might) vary. Withdrawal rates may go up or down. I think there is some value when, after years of retirement, you note that (if things have been going against you) your current inflation adjusted withdrawal is, say, 50% of your remaining portfolio because your portfolio had been devastated by a deep, prolonged recession or whatever. At that stage, looking back at a far distant initial portfolio value might be interesting but not all that pragmatic........
I look at both numbers. But it makes a whole lot of sense, as mentioned, to state which method you're using. For folks using the constant percentage method, it's always going to be the current portfolio value. For Trinity folks, I guess it would be the initial value. For folks like me who spend what they're comfortable with at the time, well, better call out whether initial or current value since we're likely looking at both.
It looks like my 2012 WR was 3.9% of my EOY 2012 portfolio value. It was 4.9% of my initial portfolio value. The relationship of the 3.9% to the "magical 4%" figure is strictly coincidental. We had some extraordinary expenses this year all of which were counted in the withdrawal rate including helping the DIL buy a new Forester, spending time with a Mercedes driving periodontist getting a dental implant, initial funding of a 529b for the grand daughter, fed taxes due to Roth conversions and deciding I deserved to drink craft beer instead of Bush Lite.........