A 12 hr max duty day seems popular, and I think that's a good idea, but with a twist. Eliminate flight time requirements and focus on duty day. We know that sit time at the airport is at least as fatiguing as time in the cockpit, so eliminate that from consideration. Instead, reduce the max duty day according to the number of legs flown, say by one hour per leg over 2. If 3 legs, then 11 hr max, if 4 legs then 10, etc.
Further reduce the duty day if any part of it will occur overnight. If any part of the duty day touches 0100 to 0500 base time, then the max duty day is reduced 3 hr. Back-side of the clock covers redeyes and high-speeds.
The duty day should also include average transit time to/from company provided hotels, just as average taxi-in time and pre- and post-flight duties are included under Whitlow. It should also include 2 hrs transit time before show. Min rest is 10 hrs, period. With the pre-show transit, min rest in base is effectively 12 hrs.
Any max duty day cannot be scheduled to be exceeded, but legal to start, legal to finish the duty day, extendable by only 2 hrs if circumstances outside of the carrier's control (IROP). The back side of the clock reduction doesn't apply if you go into it inadvertently.
Summary:
Base duty day: 12 hrs
-1 hr for each leg over 2
-3 hrs for back side of the clock duty
+2 hrs for IROP
Min rest 10 hrs, which doesn't include average hotel transit time or 2 hrs of pre-trip transit.
Examples:
Turn max (2 leg): 12 hr, 9 hr if back-side. IROP: 14 and 11.
Busy max (4 leg): 10 hr, 7 hr if back-side. IROP: 12 and 9.
Regional max (6 leg): 8 hrs, 5 hr if back-side (which makes it impossible to schedule heavily front- or back-loaded high-speeds). IROP: 10 and 7.