You will be lucky to achieve true perfection when you consider some of the possible variables acting in this situation, and a fair compromise is about the best option.
We are considering a frame which may not be perfect, fork yokes and legs not true to original spec, wheels with run out, plunger units with springs differing in performance, for a start. Also any factor within the hubs and wheel rim which moves the centre of each away from their design position....think incorrect rim offset on a respoked wheel, as an example.
In an ideal world both wheels will track with the centre of their their vertical planes in true alignment. A twist from vertical when viewed from behind produces a contact patch for the rear wheel offset to one side..... Suspension and/or frame problem. Rear wheel vertical but running to one side "crabbing" described by Jules as trapezoid, this is a matter of adjusting the rear axle each side fore and aft.
This will affect the chain alignment, so maybe best to start by adjusting the rear axle so the chain lies in its best alignment between the sprockets, in other words sprocket faces parallel as best you can, top run of chain leaving the rear drum parallel to the backplate. Adjust the chain for the best running tension with the bike off the stand and loaded as in use, setting the wheel by eye and giving it a go. Problem here is you need to be followed and observed by someone who knows what to look for, and a less than perfectly central rear mudguard can fool the brain.
In practice, tensioning the chain and matching the gap each side between the wheel rim and the lower frame member a " does a finger fit?" test is the usual cheapskate way, and is adequate for everyday riding. Certainly good enough until you get a second opinion. If it looks right, that's good enough.
Footrest bolts as a datum you would think to be the same both sides, but beware of that bend from long ago and also the possible less attention to detail in 1950's Small Heath Frame Department.....may not have been "perfect " from new, but acceptable for the market.
Swarfy.