what does the audit actually represent?
what I prefer to do is to have a transactions table and a transaction type table
the transaction type table identifies a code a descrption and its effect on stock
using the transaction code, ou know the effect it has on stock through a join multiply the value * effect
code Effect Description
WST -1 Waste
SLS -1 Sales
PUR 1 purchases
SRTN 1 returns form customer
PRTN -1 returns to supplier
in your case you need to go down that sort of route OR store the +- in your audit value. reading between the lines at present he audit value makes no sense as it doesn't distinguish between + or - values.
I'd rather be riding on the Tiger 800 or the Norton