You post is confusing - your are saying you want the field read only but
then you talk about editing it. "Read-only" by definition means you would
never even be ABLE to edit the entry in Field 1 from $100 to $80 in your
example - you simply cannot edit the contents of a read-only field. It
sounds more like you want to validate the entry in Field 1 based on the
contents of Field 2 and Field 3. While there is no direct way to do that,
there's probably an indirect way through the use of VBA. I wonder if it
would be worth the trouble to develop it though. A far easier way would be
to use a 4th field and make it a graphical indicator field that would be
green if F1>=F2+F2 and red if not, alerting you that you need to fix the
problem by hand if the condition is violated.