Help with formula
 

I trying to creat a schedule as to when a part is scheduled and completed
it's sent to assy then subtracted from the schedule. Which works fine on the
original schedule but once that qty is met I need it to some how START AGAIN
using the same schedule.
B3 has the qty
C3 has this formula =IF(B3="","",MAX(B3-SUMIF('Sent to
Assembly'!A:A,A3,'Sent to Assembly'!B:B),)) Which counts down untill the
origional qty is met
After the origional qty is met I want to be able to add more to B3 and count
down again untill that qty is met.
There is a date in columnD which I may be able to use that to seperate one
from the other.
I'm really stumped on this and really need all the help I can get! Thanks in
advance!!