Help With Simple Validation..

Hi all, i run a breakers yard and have made a small invoice / order database to make life more organised for me in access.

Anyway it's quite late and i'm v.tired, i have literally one last thing to do to complete it, if you view the screenshot attached i have:

Quantity Unit Price Discount Total

example:
Quantity Unit Price Discount Total
2 £222.50 10.0% ?

How do i work this out, i mean i can do it in excel, but where & how do i input the codes?

I was thinking:
Total=Quantity*UnitPrice-(UnitPrice*Quantity/100*Discount)=Total

ie:

Total= 2 * £222.50 = £445 -(£222.50*2/100=£4.50*10= £44.50)

How do i input this in so that the total price is worked out and will change as and when i change any of the: quantity/unitprice/discount?

Any help much appreciated.

Tom