You guys are (forgivably) thinking like military instead of like airline. Your USAF "primary" vs "secondary" time have nothing to do with "PIC" vs "SIC."

If you signed for the aircraft--if you held the "A" code--you were PIC.

I used the 90/10 rule. I made an estimate that 10% of my time as an AC ("MP" in my community) was under the supervision of an IP (i.e., someone else had the "A" code). ALL of my IP/EP time went in as PIC; 90% of my MP time was PIC. 10% of my MP time + all UP time + total of my MC time - MC "other" time went in as SIC. MC "other" time + all stray time no captured above went in as "civilian other"--i.e., part of "Total time" but not included in other areas.

I clearly marked the 10% as an estimate (as with all other areas I had to estimate), and was prepared to explain everything I'd done. I was asked exactly zero questions about it at four different interviews.

The single-seat guys have this easy. Everyone else will have to adjust this to fit their own community and their own experiences, but, this approach has worked for everyone I've known to use it.