Depreciation when business use changes each year?

While I (believe) I know how to depreciate something that is used 100% for business or whose less-than-100% use is the same each year, I'm wondering how you're
supposed to do it when the business use percentage changes every year.
Say I had 5-year property places in service in Q1. I paid $10000 for it and it was used 100% for business. I could go to page 71 of the 2014 Pub 946 and read off the percentages: 35%, 26%, 15.60%, 11.01%, 11.01%, and 1.38%. So in the initial and subsequent years I'd take depreciation of $3500, $2600, $1560, $1101, $1101, and $138 for a total of $10000 and zero remaining basis.
If used at a constant 80%, then it is my understanding I'd apply the same percentages to $8000 (80% of $10000) and so end up with $2800, $2080, $1248, $881, $881, and $110 for a total of $8000 and $2000 remaining basis.
(While typing that a new question came to mind -- what happens to that $2000 basis if the use subsequently shifts to 100% (or anything more than 80%)? Is it "trapped" because you hit the end of the property depreciable lifetime without depreciating away all the basis? Can the remaining basis be somehow considered to be "put into service" when the business use increases (like when any other previously personal asset is put into service for business use)?)
But say you had the same item but instead of the business use being 80% the whole time it changed from year to year. Say it goes 90%, 70%, 85%, 75%, 65%, 80%.
Now what happens?
Do you compute what the depreciation would have been if it was all 100% use and then multiply each of those years by the business use percentage that year? For example, would you do: $3500 * 90% = $3150 $2600 * 70% = $1820 $1560 * 85% = $1326 $1101 * 75% = $ 826 $1101 * 65% = $ 716 $ 138 * 80% = $ 110
for a total of $7948 of depreciation and $2052 of remaining basis? Or do you do something else?
For example, on page 41 of the 2014 Pub 946 I found the algorithmic description of 200% DB MACRS, and that says for the initial year: 1. Muliply your adjusted basis in the property by declining balance rate. 2. Apply the applicable convention and then for subsequent years: 1. Reduce your adjusted basis in the property by the deprection allowed or allowable in earlier years. 2. Multiply this new adjusted balance by the same declining balance rate used in earlier years.
with the overriding instruction to switch to straight line when it gives the bigger deduction.
But how does that work with a time-varying business use percentage? What initial adjusted basis do you use? How do you account for each year's actual business percentage? And what basis do you use at each step?
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Rich Carreiro snipped-for-privacy@rlcarr.com

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On Monday, April 13, 2015 at 8:55:04 PM UTC-7, Rich Carreiro wrote:

I have nothing to cite, but I'll tell you how I (used to) do it:
In your first example, using the table factors, I would compute depreciation as you did, then take the business use percentage (if less than 100%) of each year's calculated amount as your deduction. As to the amount of remaining basis that is therefore "trapped," I have always taken the view that this is basis allocated to PERSONAL use, and therefore isn't further useable for a business deduction. But I've seen commentaries that suggest differently, so who knows.
In your second example where you are computing declining balance depreciation by formula (rather than table factor), I would follow a similar approach: Calculate the annual depreciation amount based on 100% use, and then take the business use percentage of that as your deduction. But again, no "cites" on that.
If you post this question in the Taxprofessionals group on Yahoo, there is a guy who pretty much specializes in depreciation issues, and I'm sure he could expand on this...and maybe completely disagree with everything I just said. ;-)
MTW
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