We do something similar a learning cafe where we discuss a topic for an hour. I think your metric should be realistic to your organization. For us we are happy with 5% because it is difficult for us to pull alot of those people that are directly working with customers. If your people have the capability to attend these then it might be more feasible for you to get 15% or more. We also do a survey after the event to see what our people thought of it. We use a 1-5 star rating system for overall satisfaction and net promoter and then have a few open ended questions for them to provide more detailed feedback.

Hope this helps. 

We are rolling out some programming for employees that is optional.  The format is similar to that of lunch-and-learns (ie: one-off topics that are taught in one-hour or less). I have been tasked to set a benchmark around the registration for these optional classes and I can't seem to find an industry standard anywhere.  

In your experience, what metric would you use as a benchmark for anticipated registration in optional courses?  For example, in our most recent offering, we had 15% employee attendance. Is that high, low, average?  What percentage would you recommend as a goal for sessions in this format?