You are correct, the definition of Effective tax rate is what the issue is. For income taxes the effective tax rate is your tax on line 21 divided by your taxable income on line 15; no matter what you paid or didn't pay. It is an average of what the varying taxes are on your taxable income.
What comes after that is just to determine what you are going to pay or get refunded after filing. Your tax and rate has already been determined.
After that other taxes (self-employment for example) are added to your tax bill. Next your payments are subtracted from all the taxes you owe not just income taxes.
Another useful figure might be your average tax rate. Your total tax on line 24 divided by your total income on line 9. Your total tax that you pay doesn't change.
You are owed the refundable credits whether or not you have any tax due. Some of it in fact is used to pay the taxes owed; so you paid it just not out of earned income.
Another way to look at it, you could add the refundable credits to your total income on line 9 and divide that by your total tax on line 24. That will tell you the tax rate on all income from all sources including the federal government.
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