round tax

Hi,

i my country tax is 20%
as I add new product to my store i edit input: "price with tax".
price without tax is counted by zen cart. here is the problem:

zen cart count:
price without tax = price with tax - price with tax * 20 / 120

but i my country is tax law and i have to round 20/120 on four decimal places

zen cart should count:
price without tax = price with tax - price with tax * 1.6667

( 1.6667 = round( 20 / 120, 4) )

please, can you help me?

mistake in my post: 1.6667 should be replaced by 0.16667
(five decima places)

i hope there is only one php script which have to be edited and zen cart will count with rounded tax

my english is no good.
i hope you understand

i would realy need resolve that problem