Bargain Buy sell calculator

When I negotiated a salary or an item I wanted to buy, I was often a little afraid of this confrontation.
So I wanted to have some tips on how to do it better and not offend the person in front of me.
After reading the book by Chris Voss, famous FBI negotiator, "Never split the difference", I discovered a lot of tips and a method
I've never heard of: the Ackerman method.
This method is quite simple, but requires some calculations of percentages, difficult to do in your head.
So I created this application which I use more often than I initially thought, and this for all kinds of negotiations (car, salary, purchases, sales)