How can I create a custom filed with formula?

How can I create a custom filed with formula?

I have tried to make som simple formulas in the Customs fields, but I can't get it to work as I want. 

1. When I click "Formula" I get this message:
"Alert - Formula custom fields created or modified will not be evaluated in already existing records."

I don't quite understand the applications of this, or what it means?

2. I want to take the number in the box "Amount" and multiplacate it with a certain percentage.
This is the formula I've written: 

Abs(1,15)*${Potentials.Amount}

A very simple formula, but somehow it doesn't work for me. Anyone knows why it does not work, and how the formula should be written?




Any help would be very appreciated!!