INSTALLMENT SALE: What is the formula for converting Gross Profit Percentage to find Cost Basis and Selling Price?

[The Reefer Sneefer]

The IRS changed the rules for Installment Sale Income. We're required to fill out Part One for all the years now, my sale was so many years ago, I no longer know what the sale price and my costs are. I know the Gross Profit Percentage. What's the formula to convert that back?

This thread explains more detail of the problem: 

https://ttlc.intuit.com/community/taxes/discussion/re-errors-on-form-6252/01/1342326/highlight/true#...

@RobertG

[The Reefer Sneefer]

Resolved: TurboTax calculated the principle so I reported it as a long term capital gain.