have to summarise something in ms-access.

am using access 2003.

i have a query with the following fields. (this query has some other fields also. this gets data from 2 tables.)
"country" (has values of AU, NZ)
"status" - holding various stages in the process.(processing, issued etc)
" firstof inv value" which is the initial amount.
"sum of outstanding amt" - which is the current amount (if there's no amount this will be blank - null)
"final update date/time" - date of issue (will have data only if the "status" is equal to issued)

i have more than 10,000 rows of data :O(

what I need:

taking the total of "firstof inv value" (for each country) as 100%, I need to find out the % of the nulls(from "sum of outstanding amt" field) are for each country. (basically if the total of "sum of outstanding amt" can be calculated as a percentage, then the null's % can be found.)

need to do that for AU, NZ and then for au & nz together.

criteria: the "final update date/time" should have been 4 weeks or more ago.(>=28days) (datediff function achieves this)

I have no clue as to how to do this.. I am still a novice. please help. thnx