Hi kapillai
Case 1 is not the conservative method.
Try your case 2 & case 3 , you will find more difficulty to pass nozzle load.
Again if you go for case 1, I do not say it's wrong.

Since case 3 give more conservative result than case 1, I don't think we should call it inappropriate method. (in absence of more accurate data.Practical aspect there is no need also to search for appropriate data)