"Ideally, I would like it on the "C", as he hasn't many deductions for "A""

Except you don't get to do whatever works out better. You do what applies. You will want a link for the deduction to the fact of that Sched C business, or if there is more than one qualifying activity, of course, so that the full amount possible gets computed. It's not a business deduction; it's personal.

"By default, the program limits this SE health insurance deduction to the lesser of the total premiums entered or the net earnings from self-employment minus qualified retirement plans and one-half of the self-employment deduction on Form 1040. To limit the deduction based on the net income of a singe business activity, you must indicate the 'Form' and 'Activity Name or Number'. The program carries the limited amount to Form 1040 and carries the total premium entered, less the limited portion, to Schedule A as a medical expense."

I found these articles as reference:

https://proconnect.intuit.com/community/medical-tax-credits-and-deductions/help/generating-the-se-he...

https://proconnect.intuit.com/community/medical-tax-credits-and-deductions/help/how-do-i-enter-the-s...