Entering an unbound form control value into a new record field.

Hello. I hope this (friendly and helpful) group will forgive a very noob
question, but I learned from my (long gone) self-teaching years with foxpro
that a little good advice can save a lot of grief. I've been setting up my
tables for first time Access project (2007 on Vista), and thinking about my
first form. It will have a couple subforms in it, and is intended for
browsing as well as entering a new record. My focus at moment is on a text
box, whose value will be derived (and therefore be unbound) from other bound
controls (an autonumbered integer (the PK), a user entered date and user
chosen value from a combobox). I know how to construct the expression I
want and display it in the control.

My question: What's the proper way (which event code?) to get the derived
value of that unbound text box into the target field when a new record is
entered? Of course, I want to preserve the value of that field when the
user browses existing records, unless the user edits the date or text box
whose values are parsed into the expression, in which case, I'd want the
derived field to change accordingly. (It's intended as a unique catalog
number. Hence the incorporation of the PK.)

Many thanks for any pointer, RonL