Could you please check this thread?
Arrowverse CRT: Archangels and Angels
This thread is about abilities that should be added to archangels and angels and scaling of archangels and angels. First, let's start with the abilities that every angel has. Magic:The power of angels is magic based. "I call them ghouls." "They're formed by magic gone wrong." (Constantine...
vsbattles.com
However, assuming the facts are straight here, I believe your interpretation is correct. Ulti in base form 'matched' the AP of 'Post-Udon Prison Training Luffy', who is 6-C. Therefore, she should also be 6-C. Ulti in Zoan form was considered 'too strong for [Luffy's] Gear 3rd', which is High 6-C. Therefore, her Zoan form should be at least High 6-C.
Therefore, her base form should be 6-C, and her Zoan form should be at least High 6-C.