Just to close this off - after lots of consideration of wheels, tires, angles and offset from the above discussion - I have decided to aim for a 559 wheels with up to a 55mm tire (I'm thinking about giving the Rat Trap Pass Kool-Aid a sip), with the deciding factor being that I already have a set of nice 559s from a MTB which doesn't get ridden. But, in my search - I also discovered that there are some canti's which have enough adjustment to reach both 559 and 584 and/or add a removable boss, so I can change my mind (see: LHT 650b Brakes).

So this means that I need my FC to be at least 600mm to get to a zero toe-overlap. To get to this place I can use an HTA of 71.5 and rake of 50, for a trail of 62. This is definitely within the scatter plot where my sampling of touring bikes seem to fall.

Thanks for all the input.