My understanding is that since the process is a bit immature, the IRS is using some discretion with how people calculate their cost basis / fair market value, but they'll expect you to be consistent. I.e. if you cant cherry pick exchange rates from different exchanges or times to help you setup a better cost basis. My advice would be to start a very thorough spreadsheet to track your income if you plan on making more than a grand per year.
Don't make life so hard on yourself buddy. In Coinbase just go to Tools->Reports then create either a transaction history or cost basis for taxes report. Report is well laid out and clearly shows all you've done in your account.