creating database issues

Hello I am creating a database I have 4 columns 1 is physical stock 2 is allocation 3 is actual stock 4 is on order, I put 100 in physical stock 20 in allocation then it should read 80 in actual stock using the correct formula, now in column on order I will put 20 now when I receive 20 physical stock increases by 20 using a formula, now when I ship the allocation amount physical stock increases by what is in allocation I dont want this to happen the only time physical stock changes is when allocation decreases it and on order increases it this is where I am stuck please coukd someone point me in the right direction
I await your reply