Get the records. The IRS has a record of anything reported to them on behalf of this person. The Bank(s) and credit card companies, any vendors, will also have statements to send. You likely have to pay; I had to pay for 2 years' worth, back from before the only records I could find in the decedent's own filing system and unopened mail. Who got their computer(s)? Check with their State, such as sales tax filings or unemployment reporting for their staff for those years. See if someone is sitting on their records in a box in a storage unit. You'd be surprised what I have been able to reconstruct of a person's life just from doing these tasks. Then you also have supporting evidence for what you "estimated."