Determining basis for lot partitioned in Oregon

[MarkW58]

I am trying to determine the basis for a lot I partitioned in Oregon. 

Facts: 

1. Purchased property on 3/25/2022 Included a house and cottage on a large lot. Price $390,000

2. I partitioned 1/3 of the lot in 2024 and sold it on installment sale. Total sales price $65,000. Costs to partition were $8174. Closing costs for sale $1106.

I am trying to determine how to derive the cost basis for the lot. 

I will be reporting this installment sale on IRS form 6252 and not sure where else? (cap gains or losses I suspect)

Any help appreciated. 

@ Mark 

[Mike9241]

get an appraisal.  Allocation of cost is a matter of relative fair value. There is usually nothing authoritative that would provide this info that would satisfy the IRS outside of an appraisal. Even that can be challenged but its better to have one to defend your position than nothing at all.