Hi Marc,
There isn't currently a way to set step percentage in the Javascript viewer and it isn't in our immediate plans to add it to the Javascript sample.
There are REST endpoints however that support this functionality so that you could add the buttons yourself.
- UpdateJob - Percent parameter. Where you would update the percentage of work completed on the job's current step.
- Job - PercentageComplete. To retrieve the current value.
More information is available in the installed help under Developer Kit->Developer Help -> Workflow Manager Server REST.
Thanks,
Michael