I generally subscribe to the "middle/middle" school. Middle of riser to middle of bow. Shelf ends up an inch or a bit more above, which, with a straight longbow riser, puts the balance point of the bow on the heel of the hand, where it should be with a low wrist grip. On my Forward Scout (forward riser) longbows, which have a deep locator grip that raises the wrist some, the shelf moves down closer to the middle of the bow, since the pressure point has moved up.
It's all very complicated, ain't it? And, as noted:
"The top limb may now be the bottom limb and vice versa...."
And right here you have the single greatest reason to use the truly "classic" longbow riser... narrow with no shelf at all. Finish the bow, take it out and shoot it, then turn it over and shoot it again. It'll tell you which limb should be up and which down! Designate the upper by slipping a little leather wedge in the riser or inletting a pressure button and go have fun. Mostly, we all spend too much time thinkin' and not enough shootin'...
Classic shelfless riser with leather wedge and pressure button