Calculating cost basis of land on sale of real property

[RWG1950]

Customers (MFJ) have had a lake cottage and in 2018 they also bought the place next door.
They rehabbed it and sold it in June of 2020 - but kept part of the land to have a larger buffer between the two properties.
Using the assessed value of land vs improvements from the 2018 property tax statement would give me a percentage or ratio.
Applying this percentage to the total purchase price would provide the land's total initial cost.
Prorating total land cost among acreage sold vs retained would provide the cost of each - before any other additions to basis.
Anybody think I should be doing this differently ?
Thanks for comments.

[TaxGuyBill]

That sounds logical to me.

The only SLIGHT argument is if one portion of the land is more valuable than another portion. But unless there is obvious evidence of that (for example, lakeshore property versus useless, rocky land), assuming it is of equal value is probably what I would do.

[rbynaker]

Depending on the size of the parcel you might also check the county assessment. Larger parcels can sometimes be split into different categories. I have one client with 1 acre of "house site" assessed at $100,000 and 3.37 acres of "steep slope" worth only $8,400. Probably not the case here but worth a look, especially if the records are online.

Rick