Generator requirement tool accuracy
Let's take a practical example. If we have:


Appliance A: 2kW in use, 3kW when starting
Appliance B: 1kW in use, 2.5kW when starting
Appliance C: 1kW in use, 1kW when starting


Your result would be: 3kW (Appliance with the highest starting wattage: A) + (1kW + 1kW) (Total running wattage for all other appliances: B and C) = 5kW.

But it is obvious that the worst case here (if we assume only one starting appliance at a given time, as you seem to do) is actually 2kW (A running) + 2.5kW (B starting) + 1kW (C running) = 5.5kW.

So the correct formula would actually be something like: sum of all appliances running power + highest value of {difference between starting/running powers of each appliance}. Or, to put it differently: starting power of the appliance with biggest running/starting power difference + running power of all other appliances.

Now, in case of power outage, you have a problem with this logic anyway: because obviously, all appliances will be re-starting at the same time.

Edit and disclaimer: I totally agree with Olin, and did not actually even had a look at the link you provided before answering. I assumed all required information was in the question. If it's not the case, my answer may be totally irrelevant.