If you can't figure the ratio with your property tax bill, then use your cost basis to figure the ratio

Example:

Property purchased for $100,000 and $25,000 of that is allocated to the land. That makes the structure cost basis $75,000

During your ownership you put on a new roof at a cost of $10,000 Now your new cost basis is $110,000 with the structure value at $85,000 and the land still at $25,000

You sold the property for $200,000. You need to allocate that sales price between land an structure/other assets.

Doing the math, your $25,000 originally allocated to the land is 23% of your total cost basis. (rounded to nearest whole number)

Then, 23% of your $200,000 sales price is $46,000.

So with that I would report the sales price of the land at $46,000

Then, the remaining $154,000 is allocated to the structure. If more than one depreciable asset, then the remaining $154,000 is split among those assets.