Each sheet has a bunch of different events that you can tie into.
The first suggestion (that worked) tied into the worksheet_change event. This
event fires when the user makes a change to the worksheet by typing.
The second suggestion tied into the worksheet_calculate event. This event fires
when the worksheet recalculates.
Chip Pearson has some instructions on worksheet events:
David McRitchie has some notes, too:
I think I'm figuring it out. The first solution Dave posted works if the
values are entered directly into the cells whereas the second solution works
if the target cells contain formulas that derive the cell values from
elsewhere. I need to play with this and see if I'm on the right track. If so,
no further discussion should be necessary. If not, or I figure something else
out, I'll post back here for the benefit of the board. steve.