Numeric input field and checksum calculation

The two methods I mentioned for part one are throwing up an error dialog when the data is out of range or silently converting the data to so that it's within bounds. Either way, once you validate the input for one field, you can do the same process with the other fields. Of course while the user is interacting with a field (say, "N"), it's a good idea to disable the other fields so people can't mouse over to them and interact with them. Hiding them is probably too disruptive, so placing an invisible box over them a good option there. You could also put them on different layers and then go from one layer to the next once you've validated the input. 

As for the paragraph, telling the user what you're doing is always a good idea. That way they won't be confused when they see an error message or their data is converted -- they know in advance what's going to happen.