Compute Daily Gross salary from net salary (Brussels)

Hello,
I know usually the question is the opposite, but...
I saw a job offer and they ask you to make your finantial proposal. Is for a short-term contract, so it should be a daily rate. So, I can know, that if I want X as my salary net per month I just shouild divide X/21 days aprox to get a daily rate.
The problem is that they want " all inclusive, meaning gross amount in USD". Is for working in Brussels, but I have no idea how to get this value.
I've gone through different pages, lots of them explaining taxation, but cannot find for daily rates...
Is a bit urgent issue, so any help would be reeally appreciated. Thank you very much on advance