ResourceList and contexts

I've got a site with two different contexts set up which share templates. I'd like to have a template variable that uses a ResourceList by where the parent is the [[++site_start]] context key (or similar) for each site (so admins are only seeing the resources for the site they're editing).

Is this the correct way to do it? The [[++site_start]] key seems to by set with the value of the System Setting, rather than the context key. Is there another way/more MODX way of doing this?