How to measure productivity percentage in ZD Talk?

Hi Adela and thank you for reaching out to the Technical support of Zendesk.
There are several ways to measure the Talk productivity in Zendesk.
One of them will be using the Talk Dashboard and for this you will need to click the Admin icon in the sidebar, then select Channels > Talk. Click the Dashboard tab then select the number or numbers you want to view data for.

I recommend you consult the following article: 
Analyzing call activity wit the Talk dashboard  
and especially the chapter Viewing agent activity for Talk Professional. That applies if you have the Professional Talk Version.

I hope this answer your question, and remain available if you need more details.
Regards