Calculate a price with a percentage field in Acrobat

Hi to all, 

it's my first time in fields calculation in PDFs, after several decades in usual printing files… So, I don't know where to start from. Be kind 🙂

I have four fields.

First, is the number of items I can buy, field name is Q.tà.

Second, the price of the item, field name is Prezzo.

Third, the discount amount in percentage, field name is Sconto.

And this field should be a little weird.

Because sometimes we have discounts, and sometimes not.

If I set this field as a percentage (property > tab format, maybe, don't know for sure, I use the Italian os), Acrobat automatically insert a "0.00%" text/value in the field, but I would not this, i would prefer an empty field. 

Fourth and last is the total (Totale) field, showing the amount of the items x price - discount % (if we have one).

Something like this: (Item) 10 x (Price) 100$ - (Discount - if we have) 25% = (Total) 750$

Can someone helps me with the formula?

Thank you, have a nice day, bye!