Infinite scroll really throws any logical way of tracking such things out of the water.... since the page is constantly changing sizes...personally I hate (and I do mean hate) infinite scroll... as a user.. as an analytics person... basically this design just plain sucks (and there are some sites that use Infinite Scroll that I will actually refuse to use their onsite search and find content via Google because the experience is so frustrating) ... but unfortunately, sometimes business mandates that kind of site behaviour and this will be difficult to get any sort of usable data from this implementation....
Are you using Adobe's getPercentPageViewed plugin? I would assume this would track the current page size when you click on a link, so the % will be relative to the current size of the page... but the problem is the every user will be calculated on very different page sizes even within the same experience (we all know that desktop vs tablet vs mobile yields very different % results, but at least they are mostly consistent within their own platform... not so with Infinite Scrolling). Let's say the infinite scroll loads 20 items at a time... I have scrolled enough to show 60 items (and I scrolled enough to see most of those 60 without triggering the next 20), and click on item X...
Another user (with the same screen size) has scrolled enough to show 100 items (but just barely got to the bare minimum to load those last 20 items), then clicked through to a product....
The % scrolled is going to be vastly different for the same criteria... I might very well have a higher % scrolled (merely cause I saw more of a smaller set of data) than the person who scrolled more and loaded far more content....
If you are not talking about getPercentPageViewed and instead are triggering actions when the user gets to 25%, 50%, 75% and 100% of the page... then you might want to reconsider instead of that, tracking an action each time content is loaded for Infinite Scroll, and increment a value for each time content is loaded... initial page load would be "1"... first set of content to load onto the page "2", next set of data "3", and so on... then you could compare users and get average of the number of content loads a user triggered, etc...
If the number of assets per load varies, then maybe you need to track specifically the count of products loaded to the page?
When it comes to infinite scroll, it would also be best to discuss with your stakeholders the impact that it causes to analytics, and outline what data you can provide and what will be sacrificed... make sure that you have a clear definition of what can and is tracked...
Good Luck.
