Assuming no intervening movement of money into or out of the Roth IRA account:
The Adjusted Opening Balance is the balance immediately following the $6,000 contribution. That would be the balance on the date of these contributions prior to the contributions plus $11,500. (But see below since you did add to the Roth IRA during the computation period).
The Adjusted Closing Balance is the balance in the account immediately prior to the return of contribution distribution.
What you are calculating the the rate of gain or loss while the contribution being returned was in the account, then applying that rate to the contribution being returned. If the account gained 10%, the account contains $600 of earnings on the $6,000 being returned.
However, in your case you moved an additional amount into your Roth IRA as the result of a Roth conversion during the computation period. In calculating your Adjusted Opening Balance you must add this amount (since it's also present in the Adjusted Closing Balance). So your AOB will be the balance immediately following the contributions for 2018 and 2019 plus the amount of your Roth conversion.
See CFR 1.408-11 for all of the details of the calculations:
https://www.law.cornell.edu/cfr/text/26/1.408-11
Your IRA custodian should be doing the attributable-earnings calculation for you when you explicitly request a return of contribution form your Roth IRA before the due date of your tax return. If your Roth IRA balance changes daily in an unpredictable way such as when your Roth IRA is invested in stocks, mutual funds or ETFs, you won't have the current balance needed do the calculation on the date of the distribution. The account balance needed to do the calculation will not be determined until after you have to request the distribution, so the custodian needs to do the calculation.
