Adding Amounts Per Record

Hi to all!
I face the following problem: I cannot add amounts per line!
I have one table consisting of many different columns (fields), some of them having amounts. I want to update this table by doing the following:
let's say I have the following records:
1/6/2006 A 12
1/5/2006 B 20
15/6/2006 C 21
22/6/2006 D 17
25/6/2006 E 30.
I want to use a filter (e.g. the date being between 1/6/2006-30/6/2006), sort the records descending according to the amounts and then add in another field of the table A, B or C, according to whether the total amount including the specific record is above 70% of the total or above 90% of the total. In the specific example I want the following result:
25/6/2006 E 30 A
15/6/2006 C 21 A
22/6/2006 D 17 B
1/6/2006 A 12 C
Their total amount is 30+21+17+12=80 and the percentage of each record is: E:0.375 , C:0.2625 , D:0.2125 and A:0.15. Their adding percentage is
E:0.375 , E+C:0.6375 , E+C+D:0.85 and E+C+D+A:1.
How can I do that? I calculate the percentage of each record in a query, but how can I calculate their adding percentage, so that I can use an if-clause in an appending query?
Please help!!
Thanks in advance for your time

Panagiotis