logical calculation

Hello, I bought this plugin in 5feb,2014.
First of all, thank you so much for this plugin!
I used this plugin in my site for an order form.
the idea is that I have a text field that clients have to write their city,
I want this:
If(city = ‘x’ or ‘y’ or ‘z’ ) return ‘1’ else return ‘0.8’
how can I manage fields like this?

https://wordpress.org/plugins/calculated-fields-form/