Updating two cells only with different formulas

I am doing an inventory sheet where I need to have updates on two on my cells.
For example
I have B2 as my last inventory qty.
I have B3 as my on hand inventory
I have B4 as my USED inventory

B6 to B30 are my amounts used (one per day)

I need to substract my last inventory and have the results on my on hand inventory and under my used inventory I need to have what I have used per day + the day before, so in other words on this cell I want the results to be adding up until the last day of the month. .

I want these two columns B3 and B4 to be changing columns as I keep on adding data on each day.
NEW A NEW B NEW C
Start point 600 900 750
ON HAND
USED 0
STORE SHIP SO# PL #
Day 1 3 8 1
Day 2 9 22 32
Day 3 7 13 43
Day 4 12 4 55
Day 5 23 45 21
Day 6 42 34 16
Day 7 11 12 25
Day 8 5 25 75
Day 9 9 53 34
Day 10 75 73 22
Day 11 65 22 76