Don't.
You shouldn't store calculated data in a table. It's a waste of storage space
and defies normalisation. Read up on normalisation, it's not just for
advanced users as i used to think.
You should calculate this data only when you want to display it, eg in a
report, on a form or in a query. If you end up using this calculation alot,
it might be worth learning how to write a VBA function to do it. Then you'd
only have to call the function each time you wanted to run the calculation.