My take would be,
1) determine the required DoF so as to ensure that the subject falls entirely within the plane of focus.
2) lens used must be REALLY sharp within the focal plane. **this is where the FA Ltds shine, including certain other lenses including Siggy 30/1.4, 50/1.4, 85/1.4**
Why is f/5.6 more "magical"? Because for those that has lenses with distance scale, take a look at it... On a crop sensor, most of the time, the subject distance is/are between the maximum distance and infinity. Whereas on a FF, f/5.6 literally ensures that the subject would "always" fall within the DoF/focal plane.
e.g. My 28mm shows up to 3m/10feet before infinity.
Therefore, by looking at the aperture marking, setting my aperture @ f/4 for my shot would mean that my focal plane/depth of field is between 3m and infinity.. -_-"....
One reason why FF is more 3D... Because using the same lens enable the user to maximize the flexibility in DoF over a given distance...
Whereas if you take a look at longer lenses, e.g. FA 77:
1) It's sharp especially stopped down
2) It's 6m on the distance scale before hitting infinity. Therefore, you have somewhat useable aperture from f/1.8 to f/10 before hitting infinity. One would definitely be shooting at aperture wider than f/11..
BUT, taking that into consideration, use that FA77 for a full length body shot and woops! That 3D effect would not be as strong... One would then need an EVEN longer lens to extend the range of useable f/stop before infinity.
If you understand what i mean, you'll understand it would mean, using a crop sensor, one has to "somewhat" use x1.5 FL to achieve a somewhat similar effect, composition and perspective if on FF. And that means standing somewhat x1.5 further in comparison too..