I estimate that about 95% of the time I fly, I have the A-code. There's no way to know exactly what the real percentage is, without keeping a separate logbook, so I just took my best guess. My PIC calculation, then, looks like this:

(Primary + Secondary) * .95 + Instructor = Total PIC

The leftover 5% gets added to my SIC. It's an easy method to explain and justify in any interview. I've never had anyone take issue with it. Whatever way you decide to do it, make it easy to explain and understand.