SQL & PL/SQL :: Running Total SUM (Amt) Over?

My example: I'm given an Allowance throughout the week. It happens to be 10 dollars but it can vary from day to day.I can create a running total with SUM(Amt) Over etc...This is the CUMUL column in the example below.

On certain days I've spent different percentages of the allowance. (The SPENT Column which is a field in the database)I can't manage to create the AMTLEFT column in the example below.The AmtLeft column seems to be a kind of running total that 'refers to itself' so this is where I'm stumped.

Week,Day,Amt,Cumul,Spent,AmtLeft
1,Mon,10,10,0%,10
1,Tue,10,20,50%,10
1,Wed,10,30,0%,20
1,Thu,10,40,0%,30

[code]...

My imaginary SQL would look something like at this point (if I have it right):

SELECT Week, Day, Amt, Sum(Amt) Over (Partition By Week, Order By Day) AS Cumul, Spent FROM AllowancesTable

How to get the last column AmtLeft?