ETTR - How far with dark subject?

I am wondering just how far you are supposed to take exposing to the right when shooting a dark subject in a dark environment.

I understand that in most normal situations, if you practice ETTR, then your historgram should reach right up to the right end of the spectrum, without pixels actually hitting the end point (if possible). What happens when the scenario is basically quite dark and the histogram would normally be bunched up towards the left side. How would you practice ETTR in such a situation?

Thanks.

Steve