It depends on what you're trying to do. The answer is "Yes, you can have a field in a table that stores a calculated value. But you shouldn't." The correct way to do it would be to create a query on the table and have that find the total, or to create a report on the query and calculate the total there.
But if all you are doing is creating a single-page grid where values are filled in and a total is calculated at the end, perhaps you should consider a spreadsheet instead of a database?