The answer to your question is: you use your AGI. But, you're working on the wrong issue.
You stated" "My child is claimed by her father (divorced).....and I have always been the custodial parent and used her for head of household status, etc"
You can't do that anymore, now that the child "is 20 years old and ..... living on her own". She is no longer in anyone's custody if she is living on her own. Even if "living on her own" is nothing more than being away at school (only a temporary absence from the parent's home), at age 20, she is emancipated*. An emancipated person is not in anyone's "custody". The parents can no longer split the tax benefits. Only one of you can claim her and probably neither.
You may be able to claim her (if she is temporarily away from your home and technically still living with you, but not in your "custody" ) under the qualifying child rules or the father may be able to claim her under the qualifying relative rules, but not both. If her unemployment and other income totaled more than $4300, the father cannot claim her.
See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
*except in Mississippi
