I do not determine vessel selectivity based off of anything other than catheter location due to the fact that codes 36215-36218, 36245-36248, and even the cerebral angiogram codes 36222-36228 specifically state that they are for capturing "catheter placement". Because of this inherently being a part of the definition, I educate providers to understand the correct code is selected based off of the highest order vessel reached within each vascular family via catheter, and not wire or sheath.
Examples:
36245 Selective catheter placement, arterial system; each first order abdominal, pelvic, or lower extremity artery branch, within a vascular family
36223 Selective catheter placement, common carotid or innominate artery, unilateral, any approach, with angiography of the ipsilateral intracranial carotid circulation and all associated radiological supervision and interpretation, includes angiography of the extracranial carotid and cervicocerebral arch, when performed
I've run into the same issue as you though, that I would feel more comfortable if I had a document I could cite from a reputable body to back this up. I recently went on a deep dig search for one, as I have a provider who I need to get up to speed on lower extremity angiograms, and I have yet to come up with one, after reviewing CER, CPT Assistant and CPT instructional forwards.
I would explain to your provider that since the code definition explicitly states that these codes capture "catheter placement" that whatever level of selectivity reached with a catheter is what you should safely code to, and nothing else.