Unbound text box calculation

Thanks in advance for the assist. My mind went numb...

What must I do to have an unbound field only perform the calculation for the
current open record that I am working in and not show data in a new records
same unbound field when I go to the next record entry?

I have an unbound text box on a form - call it Box C. In it's properties
control source I have a calculation (i.e. =text box A input / text box B
input) that I wish to show within the form prior to moving to the next
record. I then wish this field to recalculate or return null (if box A and/or
B are empty). Box C will calculate correctly, but when I move to the next
record to input data, Box C maintains the previous record's data - but only
within Box C. All other fields are blank and ready to accept new data.
Box C calculates within the new record based upon the prior records data in
Box A and Box B; even when I have not entered any data into the new record.
Please point me in the right direction. Thanks