Cost basis for subdivided land + house

[Jonathan Joshua]

I recently bought 11.3ac with a house for $60k.

The county tax assessment is $80.6k for the land and $16.8k for the house.

I am subdividing and selling 2.5ac with the house and keeping the rest of the vacant land.

The specific 2.5ac that I am selling was appraised for $28k.  The house was determined to have zero value and is being sold as-is, where-is along with the land.

What is my cost basis for the subdivision?  For tax purposes, I'd like it to be as high as possible but I'm not sure how to get there.

The easy calculation is to use the tax assessment to split the $60k purchase price between the land and house and then do the math.  $4399 per ac x 2.5 is $11k plus $10.2k for the house.

However, I can clearly make the case that the specific 2.5ac that I'm subdividing is more valuable than the rest of the vacant land based on the fact that it is flat and cleared and has a driveway vs the rest of the property that is not flat and not cleared.  Also, the appraisal.  But what numbers would I use?