Then play it safe and use a basis that you can prove, which would be the cost basis of the entire property when you inherited the first half 15 years ago. Your "gain" will still be less than $250K and since you qualify for the "lived in 2 of last 5" years you owned it, you won't pay any tax on the gain.
If you're married and your spouse also lived in the house with you for at least two of the last five years you owned it, (with both names on the deed of course) then your first $500K of gain is tax exempt.
Now if the difference between the cost basis you can prove, and the cost basis you estimate, doesn't put your overall AGI in a higher tax bracket, I'd go with the total cost basis I can prove and leave it at that. Does this make sense to you? (it does to me, but that don't mean anything.)
