Change width of WKInterfaceSeparator

I would need to adapt the width of WKInterfaceSeparator in a table depending on some cell characteristics.

I have tried to set the width property with

separator.setWidth(10.0)

where separator is the IBOutlet for WKInterfaceSeparator

But this has no effect.

I have found an interesting workaround

https://stackoverflow.com/questions/33923997/apple-watch-table-design

by inserting a group.

I would prefer to set directly. Is it possible ?