Hi accountants... I am not, and need a view on VAT that is driving me nuts!

I am a web developer, currently working on a high end commerce system. All prices within the database are net.

So, lets say an item is £5 gross, that is 4.17 net.

4.17 x 1.2 = 5.004, rounded is 5. lovely.

2 items at 5 would be 10 as expected!

However, 2 x 4.17 = 8.34 x 1.2=10.008, rounded is 10.01

is it correct to add them gross and then calculate the difference (being VAT), or should they be calculated net with vat added (getting me an annoying extra 1p)?

I need to lose that 1p or else customers are going to start complaining!

Thank you for your views in advance...