Validation Rule sharepoint list

hi there,

I have two fields

net amount and

vat amount

what i want to do is the "Vat Amount" field not to accept MORE than 24% of "net amount" field...

for example.... if user puts 100 euro on "net amount" field when the "Vat Amount" should be NOT more than 24 euro... Less than 24% is possible... more must be NOT allowed...

is that scenario could be implemented to a sharepoint list? could u please propose me some general guides to follow?

thanks in advance