Calculation on previous columns

[sunil-kumar5]

Hi Team

below is my data model

and based on that I need to create a table per below

username	targetmonth	sum(invoice_value)	amount	calculation
A	Oct	76000	50000
A	Nov	100000	200000
A	Dec	76000	50000

Now I need the value in calculation column and the logic is

if(amount > sum(invoice_value), sum(unit*price),'NA')

now the thing is sum(unit*price) should only calculate unit and price when it start meeting the amount from the sale date , lets say user meet the amount target on 15th of dec than sum(unit*price ) should start after 15th not for all days.

for dec the total sum(unit*price ) is 7000 however after meeting the amount it should start after 15th dec and come to 4000

I am not sure if I was able to clear it out

Thanks