<input data-custformat="##.##\\%" value="12"/>
fails to render the format correctly. The output looks like 1200\%, as the double backslash is escaped to a single backslash, and the field maintains the default Kendo percentage behavior of multiplying the field's value by 100.
<input data-custformat=@(@"##.##\%") />
formats the number correctly, and overrides the default Kendo percentage behavior. Notice that in the string literal, only a single backslash is used.
This is a somewhat niche and subtle issue, but caused a bit of frustration while trying to integrate Kendo UI with ASP.NET MVC.