You could use that method, except that the nominal voltage these days is 240. That would give you the minimum size generator that can handle the same total load that the service panel can handle. But the loads inside the building are likely to be much lower than that. And the loads that are needed, in the event of a loss of utility power, are lower still. So I would do a load calculation, and I expect that would tell me that a smaller generator would suffice.
That said, if your customer is willing to pay any price for the assurance that 100% of the building will have backup power, then a 50kW generator would certainly give the customer that assurance.