http://cutroni.com/blog/2014/02/12/advanced-content-tracking-with-universal-analytics/
Adapted for Google Tag Manager by Simo Ahava
For this code to work, you will need the following:
TRIGGER
Create a new Custom Event Trigger, and set Event Name to scrollEvent.
VARIABLES
Create the following Data Layer Variables:
{{scrollCategory}} - Variable Name: scrollCategory
{{scrollAction}} - Variable Name: scrollAction
{{scrollLabel}} - Variable Name: scrollLabel
{{scrollValue}} - Variable Name: scrollValue
{{scrollNonInteraction}} - Variable Name: scrollNonInteraction
{{scrollMetric1}} - Variable Name: scrollMetric1
{{scrollMetric2}} - Variable Name: scrollMetric2
{{scrollMetric3}} - Variable Name: scrollMetric3
{{scrollDimension}} - Variable Name: scrollDimension
EVENT TAG
Create new Universal Analytics Tag of type Event, and set the following fields:
Category - {{scrollCategory}}
Action - {{scrollAction}}
Label - {{scrollLabel}}
Value - {{scrollValue}}
Non-Interaction Hit - {{scrollNonInteraction}}
Custom Dimensions
...set dimension index... - {{scrollDimension}}
Custom Metrics
...set metric index... - {{scrollMetric1}}
...set metric index... - {{scrollMetric2}}
...set metric index... - {{scrollMetric3}}
CUSTOM HTML TAG
Create a new Custom HTML Tag, set it to fire on the pages where you want to track scroll depth, and copy-paste the code in script.js into it. Remember to change the contentElement variable with a selector that matches the main content area on your site.