Needing help for a NetPromoter calc rule in an element

[studioflux]

Hello.

I'm trying to create a netPromoter score form in my website.
It's a feedback app for my customers. They fill a form with a note (between 0 and 10)
if the customer answer is 10 or 9, the score is +100
if the customer answer is 7 or 8, the score is 0
if the customer answer is 6 or less, the score is -100

I have to calc the average of all these scores.

I created a radio button element with the 10 values.
I need you to help me on the calc rule, to obtain my score under the table.


Thanks for your help !