> I was describing the difficulty in calculating a profit
> rate, in part because of the inability to get a handle on
> aggregate depreciation.
My point is that it doesn't matter how you do it, so long as you do it consistently among the rates you are calculating/comparing. There's no "better" or "worse" ways to do it, and there's no advantage to doing it one way or the other, in terms of profit rate.
The only advantage to be gained by shifting depreciation schedules is a sleight of hand against those who aren't paying attention. This is rather prevalent among, for instance, the quarterly-results crowd who use this kind of thing as the basis for making investment decisions[*] -- but you are not proposing that kind of use here, you're proposing a sober look at the situation using good math.
So: don't get hung up on depreciation.
/jordan
[*] As an example if I say I am highly profitable this quarter, my stock price might go up; if that extraordinary profitability is as a result of an accounting change which makes my previous quarters less profitable, and not because I've *actually* become more profitable, there's a subset of The Crowd who will notice and not participate in a rally; there are those who can not or will not to the math, and they will participate. Such is life.