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@TaxMonkey wrote:

is the business miles divided by lifetime mileage the proper measure of business use or should the business use be measured from the first time the vehicle is "converted" to business use.

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I'm too lazy to look for citations, but ...

For calculating a gain, you use the Adjusted Cost Basis.  Logic would dictate you use the time period from the original Cost to when it was sold.

For calculating a loss, you use the Adjusted FMV Basis (assuming the original FMV was lower than the original Cost).  Logic would dictate that you use the time period from the original FMV Basis to the when it was sold.

And of course there are situations where there is neither a gain or a loss due to neither of the above applying.