i don't know of a way of doing this in a table.
would it be ok to use a query to perform this action?
1create a new query
2bring in the 2 fields from your
table you want to include in your calculation
3create a 3rd column to perform your calculation.
In the Field: section (where you would normally see the fieldname) of the 3rd column you would enter something like this:
Result: [Field2]- [Field1]
Result represents what you're setting the name of the 3rd column to.
Make sure to correctly name your field references. For example, if the the field name in the table is 'FieldOne' input [FieldOne] and not [Field1].
if must needs have this in a table, you can run a make-table query using the same logic as above.
I hope this helps.