I have crypto losses though a non-US exchange and its nearly impossible to get clear cost basis documentation

[teamwork911]

I can fairly accurately calculate overall cost basis, but not each transaction.
Will I end up with an audit nightmare?

[JohnB5677]

Possibly, but documentation is critical. 

• I know the diversity of cryptocurrency, and finding some details may be difficult. 
• The first step is that you claimed it on your tax return.
• Now DOCUMENT IN WRITING the steps you went through to attempt to get the most accurate cost basis.  This means on the date you acquired it down to the minute.
• If this information is not available, describe what you did.

Cryptocurrency is kind of the new kid in town, and the IRS watches it with a suspicious eye.  You would initially get a simple letter prior to any investigation.  You would need your explanation at that time.