I think the step you are missing is the detection of the elements on page. To do this create a Custom JavaScript variable (lets call it 'Detect'):
function(){
return $('#FILTER_ID).length>0
}
This function looks for whether an element with the id FILTER_ID is present on page, and returns true or false.
From here you have a number of options.
1: Create an event on page-view if Detect equals true. Create an additional event for clicks on the element. Divide clicks events by pageview events and you have your interaction ratio.
2: Create a new hit level custom dimension. In your Pageview Tag, set this custom dimension to the value - Detect. Create an event for clicks on the element. Create a session segment where Detect equals true, then divide events by pageviews and you have your interaction ratio.