It would be best to use the original cost of the property for the percentage, since you are first dealing with the cost when your purchased it. The improvements came later on, so they shouldn't factor into the original cost percentages.
Also, since you didn't hold the house for long, it's not going to affect your taxes much either way, as your depreciation will be minimal so you won't have to deal much with the ordinary income factor the comes about from recapture of depreciation.
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