Auto entering field from another field without tax

[alekmih]

Please help,

I want to make field to insert automaticly data from other filed but without tax (18%)

example
I have filed [Iznos] where the price is with tax and I want to make another filed where data will be entered automatically and without tax

I tried in "defaul value" to enter
[iznos] / 118
but don't works.

Thanks

 

[KenHigg]

I think it would be something like [iznos]/1.18. But I would calculate it on the fly when ever you need it and not store it.