If you don't have a complete record of the home's business/rental use since you first began using it for those purposes. you will need to estimate the missing information. From whatever records you do have and your own recollection, estimate the business/rental use for each year then calculate the average.
As a check on your estimate, you might use your tax returns for the last several years. If you don't keep your returns, you can request copies from the IRS by submitting Form 4506, Request for Copy of Tax Return. Refer to the form for the processing time and fee.
Be sure to keep your calculations should you later need to prove how the percentage was calculated (unlikely).
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