As fellow Access MVP John Vinson likes to say "Storing calculated data
generally accomplishes only three things: it wastes disk space, it wastes
time (a disk fetch is much slower than almost any reasonable calculation),
and it risks data validity, since once it's stored in a table either the
Total or one of the fields that goes into the total may be changed, making
the value WRONG."

Put the calculations in a query, and use the query wherever you would
otherwise have used the table.

Calculated values should never be stored in a table. Add the calculation
to the query or the report using an expression. Check the link below for
some examples.
http://office.microsoft.com/en-us/assistance/HA011814491033.aspx

gls858