then the number will changeThe number would not change unless you put code in place to insure it changes. That's the entire concept behind NOT storing calculated values, and instead storing the root values - changes made to the "root" values would be reflected in your forms and reports and queries without you having to do anything further. If you stored the full calculated value, you'd have to insure you make the required data changes in all those locations to insure you write back the calculated value correctly.
I would rather not have to lock down the fields.I'm not sure what you mean by this, but you don't have to lock down anything (and you can't do that in Access anyway).
for the most part the only time that the other parts may change is due to an entry error.I've been doing this a long time, as has Pat, and my experience has always been that forcing non-standard behavior (like this) is never a good idea.