Good morning, Dermot-
I didn't mention the *easiest* event: orchestrate one. Advise your
users they must press the "Validate" button at a certain point in data
entry. Adding a button to a sprdsht is easy, and just as easy to
assign code to that button.

If that's amenable to your process, it's just a matter of stepping
through the required logic. Your post says
entered in a top to bottom order. They are either all required for a full calculation or only three under certain circumstances
.... and
if the code could incorporate some form of logic..cells (A AND B AND C) OR(A And B......And F)

This is absolutely do-able: if you can determine how many circumstances
are possible and what cells are required in each given circumstance,
then we can write the code to accommodate it.

How do you feel about the "Validate" button?

And just for grins, what is your continent/time zone? I am in North
America, Eastern Daylight Time.

Dave O