auto-enter sometimes?

I'm not sure there's a solution to this.

Field A displays a drop-down from a value list. Associated are three
other fields that do a lookup based on the selection in A. A is a key in
a relationship with another table.

The problem is that the user may then choose to edit field A (it's a
headline to a news story and has to fit a certain character count),
changing it so it no longer matches the key field in the other table.

One of the fields is a calculation. If I check "replaces existing
value," then editing field A causes a new lookup, no match, and the
field is overwritten with nothing. So I don't check that. But now
suppose the user decides to go back to the drop-down to make a different
selection. There's already a value in the calculated field, so it's not
updated.

What I need is a way to make the calculation work when there's a
selection from the value list but not when the field is edited manually.
IOW, I want to have my cake and eat it. Any suggerstions?
.