YO,
The 'lightning' dissipation capability of the low voltage electrical system has been greatly exaggerated.

Why is a transformer grounded, to limit voltage to ground of course. Where is a transformer grounded ? At the transformer.

We have no equipment to capture or dissipate, that is the function of the arrestors at the transformer on the pole. While it is true that some houses have arrestors, most do not, nor is it required. The rare overvoltage that comes from the neutral into the house will dissipate into the ground rod (water/sprinkler pipe, ufer..etc.)and a #6 is way more than sufficient.

The grounding electrode at the structure does not come into play at all for an overvoltage that would come across the phase conductors, that's how people get computers burned up that don't have their surge protectors on them. In other words, by that time the utility equipment has failed. If it did it very often, you would not protect it with the $20.00 protectors picked up at Wal-mart.

The real issue of grounding may be lightning, but not at the structure. As for calculation, impossible, since it isn't even designed for it ie: no required surge arresting equipment, no 25 ohm requirement only that if it doesn't meet 25 ohms you drive another rod, not make it 25 ohms or less, the utility side is a different story. So calculation based on the hypothetical strike that is not supposed to get through anyway would be a 'crap' shoot at best.