Hello Chris,
1. set you template as you like
2. Edit -> Edit XML source

Find what you are looking for and set it in JS
In your case:
<delivery ....>
<execution broadEndDate="" broadStartDate="" maxRetry="5" nextPassDate="" passStartDate=""
retry="0" retryPeriod="3600">
<controlGroup activeOrder="0" enabled="true" fixed="10" percentage="10" schema="nms:recipient"
type="1">
<where filteringSchema=""/>
<humanConfig>Random sampling with size limited to 10 %</humanConfig>
</controlGroup>
</execution>
....
</deliver>
In JS of delivery activity set control group:
delivery.execution.controlGroup.type = 1;
delivery.execution.controlGroup.percentage= 50;
delivery.execution.controlGroup.fixed= 10;
delivery.execution.controlGroup.schema="nms:recipient";
In workflow

result:
50% control gruop

Marcel