To add more Zoom functionality to the viewer, you will need to add the following to the HeadJS
var zoom_sizes = [1000, 1100, 1300, 1500, 1700, 1900];
To set the default zoom level across all platforms, add the below line of code and modify the value as necessary:
viewer.zoomSize = 1000; // fixed zoom size, either width or height of a single page depending in ImageDimension
Once it's added to HeadJS, the array can be modified with additional zoom levels; by modifying the values within var zoom_sizes.