How to set Parent Group Value according to Repeat Count?

[vivoices]

I have difficulties finding a way to update an amount element in the Parent group according to the number of repeats of its child (repeat) group.

Do I need a calc element in the parent group to achieve this functionality?
Where do I set the js event?
How do I access the repeat counter in js?

Joomla 3.4.8
Fabrik 3.4.1

Could anyone point me in the right direction?

Thanks,
Udo