Depreciation when personal-business use ratio changes

The way that makes sense to me is: Calculate the depreciation schedule if 100% use. For example if 3 year property, SL, and cost price is $6, depreciation is 1, 2, 2, 1. So first year it is 75% of $1 or $0.75; next year it is 100% of $2; etc. Then again, if you used section 179 the first year you only wrote of 75% of the asset, whereas if you change the percentage every year you can take more.

Here's what I'm having trouble with. Let's say we have 3 year property, SL, and a $100 basis.

In case one, business use is 75% for the first year, 100% business for year two.

For case two use 100% business use for all years.

Second year depreciation is greater in the first case then in the second case. In a sense the 25% non-business portion was converted to business use in the second year. But this conversion does not involve using lesser of the FMV or cost.

Minor point, where did 0.1 for first year come from? Under SL it should be 1/6 or 0.166666.

OK, let's suppose it is 0.1. Here's my way of thinking of it, based just on common sense, and not any pubs:

The first year total depreciation is ($100)(0.1) = $10, remaining basis is $90. Of the $10, $2.50 is personal use depreciation and is not deductible, and $7.50 is business use depreciation and is deductible. Or try this way: The asset is $100 is comprised of two assets -- a $75 business part and $25 personal part. The personal part has no depreciation. The business part has depreciation of ($75) (0.1)=$7.50. The remaining basis of the business part is 75-7.50$67.50.

Suppose you then sell the item for $80. Well, you're selling the business part for $60 and personal part for $20. You have to recapture $7.50 (depreciation actually taken), and you have a business gain of 60-67.50 or a business loss on form 4797 of $7.50. I think the recaptured depreciation of $7.50 may cancel out the loss, but maybe the capital loss of $7.50 is taxed at 15% and the recaptured gain taxed at 25% -- would need to look at form 4797.

And if you didn't dispose of them item the second year total depreciation would be ($100)(0.2)=$20. Since the business use depreciation is 100% all of the $20 is deductible.

Now you sell the asset for $40. Can you figure out how to figure out the depreciation to recapture and capital gain?

I guess I multiplied 3 times 2 and got 10. (I should have said 5 year SL)

This is were my misunderstanding was - feeling stupid. Thanks.

Depreciation is straightforward. I think the real difficulty is how to figure out the cost basis for the purpose of filling out form 4797 when the business use ratio changes each year.